Fix: copy all arg keys, including those set to None

[georgesittas]

The following behavior was observed after the latest __deepcopy__ changes:

>>> from sqlglot.optimizer.simplify import gen
>>> from sqlglot import parse_one
>>> q = parse_one("select x from t")
>>> gen(q)
'select _,_,_,x,_,from t,_,_'
>>> gen(q.copy())
'select x,from t'

The root issue here is that we don't copy over arg keys with None values anymore due to using the set method for leaf nodes, meaning that the original and the copied AST may now have different args dicts.

(The self.assertEqual(query, query.copy()) test passes in main - I added it to future-proof __deepcopy__, __eq__)