Logical equivalent of D3844.

foo = {'a': 2}
bar = {**foo}

Previously, foo would incorrectly be added to the value type of bar.

N.B. this allows Ast::DictAst::keys to contain null values (for unpacked dicts, where there is no key). This is necessary to determine which values should be unpacked, because the value node has no different properties.

No test regressions. Two new tests previously failed, now pass.