Behaviour when no results can be returned from a statement

[tlocke]

The DBAPI 2 spec says that the fetchall() method should raise an exception if it is called when there is a null result (note that this is difference from a result with no rows). pg8000.dbapi should probably be changed to conform with that.

We should also probably change pg8000.native's run() to return None for a null result.

[tlocke]

So now pg8000.native's run() does return None for a null result. Also, for pg8000.dbapi's fetch*() methods already raise an exception for a null result.