One may speculate on the differences of zero, null and microgravity...
Microgravity is something that only concerns the eggheads who want to know precisely
how things behave in freefall. None of our heroes should use that term when talking about spacewalks or spaceflight, really.
It would be fun to interpret "null gravity" as the artificially induced state of freefall, that is, the negation of natural pull with technology, and "zero gravity" as the absence of pull. Starships would typically have "null gravity" if things started to float around when the ship remained in motion, as freefall would be an unnatural way for a starship to move. But the immobile DS9 might have "zero gravity" when Melora turns off the gravity plates. I doubt the terminology is in any way consistent in Trek, though. Oh, well.