Is tomato sauce safe if the water evaporated in the canner?
If the water was boiling the whole time, you are fine. 

There is research-supported (finally) evidence that "Steam canning" is actually just as effective as boiling water bath canning, if done properly. Unfortunately, the folks that have been insisting that you must have everything submerged are rather slow to change recommendations based on the research, after decades of claiming that they didn't recommend it since there was no research. It makes me wonder what in heck their actual agenda is.

In any case, if your boiling water bath was boiling the whole time, the portion of the jars that was "steam-processed" would have been steam processed properly, so you're good.

Note that while the research was aimed at the "steam canner" pots, a partially full boiling water bath in a normal pot is exactly the same above waterline when the lid is on.