Treat maps with falsey keys as non-empty

[vitalivo]

DotMap({0: "value"}).empty() returns True because any(self) tests whether any key is truthy. The same happens with False, an empty string, or None as the only key. As a result, addition treats these populated maps as initialization placeholders and returns the replacement value instead of raising TypeError.

Check the mapping length instead. Add regression cases for each falsey key, including the addition behavior and preservation of the existing entry. All four cases fail before the fix.

Validation: the full python -m unittest suite passes on Python 3.10 and 3.14.