q1: can u build on null?
Yes. The first tile (0-0) you can build on it, but not walk on it. If you find a tile you can both build and walk on it. let me kno!
q2: can you create a building on null?
Yes. Explained abouve, only you do not need to be able to walk on it.
Q3: if you can create a buildin on null how?
Triggers, or preplace them.
Q4: Simmular to questions 2 and 3. Is their a way to make a unit do an ability to create a unit on null?
You know the 14 creep tiles? the two after them you can walk on. If you put the nulls after a dirt or other walkable terrain, and then put a zerg building with creep you can walk onto the null, but the unit acts as if its walking on invalid terrain.