eliminate_item_equal() uses quick_fix_field() for Item objects it creates.
It computes some of their attributes on its own (see update_used_tables()
call) but it doesn't update not_null_tables_cache.

Recompute not_null_tables_cache also. Not computing it is currently
harmless, except for producing MSAN error when some other code
propagates the wrong value of not_null_tables_cache to other item.