Looking at maps or globes today, every inch of land is directly controlled by one country or another. The vast emptiness of the Sahara is claimed by a handful of countries even though the benefits to their country the desert brings is negligible. This goes hand in hand, I think, with the ideology of solid borders overtaking that of a march or neutral zone.
There is a small area between Egypt and Sudan which remains unclaimed, and another area between Burkina Faso and Niger which was unclaimed until last year.
Does anyone know a good flag maker?
Not exactly sure how to answer your question.
The short answer is that there is no more terra nullius because states claimed all of it. Geopoliticians around the turn of the last century—Halford Mackinder most prominently—termed this the "closure" of the world, insofar as there were no more areas in which one could expand without having to take territory from another state.
Even the notion of terra nullius is held to be Eurocentric and imperialist. There were certainly native peoples in all of the land that European colonial powers treated as terra nullius—e.g the Americas and Africa—it's just that those indigenes were not regarded as civilized or sovereign.
It's also not unacceptable to have areas unclaimed by a sovereign state, but it's unclear what the practical value of relinquishing sovereignty over a tract of land would be. In general, states seek to expand the areas and resources under their control—e.g. see the current dispute over the Spratly and Paracel Islands in the South China Sea. Why would a country give up territory uncontested, even if it were economically worthless?
I'm not sure that 'borders' qualifies as an ideology, but I can certainly see the spread of sovereignty or the sovereign state as an ideological shift that accompanied the expansion of European international society across the globe via colonialism.
It's worthwhile to point out that most of the land in Antarctica has been claimed, these claims simply aren't widely recognized because of a lack of any permanent population.
If you don't get an answer, you might try and X-posting for the International Relations folks over at /r/AskSocialScience.
I believe an actual Naval Historian might have some insights into how Maritime/Economic Exclusion Zones and fishing/mineral rights played a hand in prompting nations Into claiming every little piece of land that sticks up out of the water.