The question of whether Caesar could be considered anything like "emperor" is a fraught one. That is but one of many threads tackling the same question--there's a reason that both ancient and modern commentators do not generally consider Caesar to have been an equivalent to the emperors of the Principate. Indeed, they don't even describe Caesar's rule as dominatio, for various reasons.

Sidestepping that issue, we find in fact that the question of Caesar's guards has nothing to do with it and is in fact quite simple: in March of 44 Caesar no longer had a bodyguard. Our sources are unanimous in pointing this out. From Appian:

σπεῖραι δ᾽ ὅσαι στρατηγίδες αὐτὸν ἐκ τῶν πολέμων ἔτι ἐσωματοφυλάκουν, ἀπέστησε τῆς φυλακῆς καὶ μετὰ τῆς δημοσίας ὑπηρεσίας ἐπεφαίνετο μόνης

Praetorian cohorts guarded him during wartime, but this guard he dismissed and he would present himself with only his apparitores.

Apparitores are nothing more than the regular civil staff of a magistrate: scribes, lictors, heralds, etc. The decision to be accompanied only by the symbols of normal magistracy was a conscious one, as Plutarch relates:

τῶν δὲ φίλων ἀξιούντων αὐτὸν δορυφορεῖσθαι καὶ πολλῶν ἐπὶ τοῦτο παρεχόντων ἑαυτούς οὐχ ὑπέμεινεν, εἰπὼν ὡς βέλτιόν ἐστιν ἅπαξ ἀποθανεῖν ἢ ἀεὶ προσδοκᾶν

And when his friends thought he should have a bodyguard [the word is derived from "spear-bearers" and in Greek parlance is quite specifically an armed guard usually associated with tyrants] and many of them offered themselves to him he did not permit them, saying that it was better to die once than always to be expecting it.

Caesar's pithy comment aside (quite in character with his general trust in dignitas), Plutarch's inclusion of this line in a list of civil instances of Caesar's well-known clemency (another result of his habitual obsession with dignitas) makes it clear that he thinks of this as an item characteristic of this habit. Suetonius thinks basically the same thing, although he offers other possibilities in his fashion:

Suspicionem Caesar quibusdam suorum reliquit neque uoluisse se diutius uiuere neque curasse quod ualitudine minus prospera uteretur, ideoque et quae religiones monerent et quae renuntiarent amici neglexisse. sunt qui putent, confisum eum nouissimo illo senatus consulto ac iure iurando etiam custodias Hispanorum cum gladiis †adinspectantium se remouisse. alii e diuerso opinantur insidias undique imminentis subire semel quam cauere < . . . . . . > solitum ferunt: non tam sua quam rei publicae interesse, uti saluus esset: se iam pridem potentiae gloriaeque abunde adeptum; rem publicam, si quid sibi eueniret, neque quietam fore et aliquanto deteriore condicione ciuilia bella subituram.

Caesar left to certain friends the suspicion that he wanted neither to live any longer nor that he cared, because his health was declining, and that therefore both what the omens warned and what his friends related to him he had ignored. There are those who think that trusting [so much] in the most recent SC and the oath of the senators he had even dismissed the guard of Spaniards guarding him. Others, however, think that he preferred to walk with plots hanging over him once and for all than to worry all the time. And some say he used to say: that it was not as much for himself as for the state that he was healthy, that he had taken up long ago ample power and glory, and that if something were to happen to him there would be no peace and that the state would be subjected to civil wars in a rather worse condition [the text of these last couple sentences is corrupt]

Some of what Suetonius says seems to be historiographically problematic (is he being teleological with the benefit of hindsight there in the last bit?) but the testimony that Caesar dismissed his military guard, as was expected of a magistrate in the city, is there again. Cassius Dio mentions that the senate apparently voted him a guard of equites and senators (like all of Cassius Dio a lot of this passage is problematic and clearly confused), but even he notes that Caesar dismissed his formal guards:

ὅπερ που ἐγένετο, καίτοι τοῦ Καίσαρος καὶ δι᾽ αὐτὰ ταῦτα θαρσήσαντος ὡς οὐκ ἄν ποτε οὔθ᾽ ὑπ᾽ ἐκείνων τοιαῦτά γε ψηφιζομένων οὔθ᾽ ὑπ᾽ ἄλλου τινὸς δι᾽ αὐτοὺς ἐπιβουλευθησομένου, κἀκ τούτου οὐδὲ σωματοφύλαξιν ἔτι χρησαμένου: τῷ γὰρ δὴ λόγῳ τὸ πρός τε τῶν βουλευτῶν καὶ πρὸς τῶν ἱππέων τηρεῖσθαι προσέμενος, καὶ τὴν ἐκ τοῦ πρὶν φρουρὰν προσκατέλυσεν.

And thus it was, and though Caesar was confident even on account of these measures [his various honors] that he would be plotted against by any one of those who had voted him these things nor, through them, anyone else, and as a result of this he did not any longer make use of his bodyguards: for in principle he had accepted being guarded by senators and equites, and he dismissed the guards previously about himself.

Like most of these accounts there's some weird teleological stuff going on here and a lot of stuff entering the narrative probably from later "legends" of Caesar's life. That's not what this discussion is about, though: our sources all say he dispensed with his guards, as part of a general move to dismiss the trappings of the military and return to his guise as a magistrate. The timing isn't always entirely clear--some don't give any timing at all, others place it after his honors or after the attempt by Antony to crown him at the Lupercalia--but on Dec. 21 of 45 he still was accompanied by his troops: in Att. 13.52 Cicero mentions that at a dinner party Caesar attended near Puteoli Caesar was accompanied by so many troops that he had them pitch a camp in the field near the villa. Caesar had only arrived in Italy from Spain recently (I want to say Sept?) and he probably dismissed these troops not long thereafter