The Court Martial can't "nullify" it. They can find guilty, not guilty, or a couple of specified other ones (not guilty only by reason of lack of mental responsibility for example). They can, of course, find "Not Guilty" if they want. I don't believe the Judge can impeach a finding of "not guilty unless there is one of a couple specific reasons*. Does that ever happen? Maybe, but I've never heard of it. What are the chances? Effectively zero. If at all possible you want to avoid a Court Martial. It is not set up to favor the accused even a bit.
I am not a JAG officer, or even have much experience with the process. I've only seen one soldier go all the way to GCM in my entire career, and that was mostly stupidity.
*From the Manual of Court Martial:
Rule 923. Impeachment of findings
Findings that are proper on their face may be
impeached only when extraneous prejudicial
information was improperly brought to the attention of
a member, outside influence was improperly brought
to bear upon any member, or unlawful command
influence was brought to bear upon any member.