You can read the full House and Senate proceedings at the Yale Law School.

The short answer is the Senate only voted 82-0 (not 88-0, that was the tally for Germany) for the vote against Japan, because there were only 82 senators present. There were no abstentions. You can see which senators didn’t vote here. But I haven’t been able to track down why those senators were missing. It should be noted however that the senators were clearly returning to Washington. The declaration of war on Japan only had 82 senators. The declaration against Germany three days later had 88. And the declaration against Italy later in the day after Germany had 90.

None of the Senate votes had any abstentions.

It’s interesting to note that’s not true of the House. The House had a single No vote, Jeannette Rankin of Montana. There were also 30-36 abstentions on the various declarations. There was a strong pacifist movement in Midwest and Plains states. House leadership was able to get most of the pacifists to vote “present” rather than “yea” or “nay.” But they weren’t able to get Rankin to abstain.