Make Cursor.execute() return None instead of -1

[yzchang]

Return a None instead of -1 when we can't determine a meaningful row count for the result of a Cursor.execute() call, e.g. for the DDL statements.

For reference, see "Cursor attributes" > ".rowcount" (https://peps.python.org/pep-0249/#id20)

PS> Jupyter notebook shows "-1" for a Cursor.execute('create table ...'). Need to find out where exactly the '-1' comes from.

[joerivanruth]

Fixed in 1.7.0