fix: treat empty property strings as present in helpers

[daviddallakyan2005]

Closes #3713

Rationale for this change

property_as_int, property_as_float, property_as_bool, and get_first_property_value used a truthy walrus check (if value := properties.get(...)). An empty string is falsy in Python, so a present "" was treated as a missing key: the typed helpers returned the default instead of parsing, and get_first_property_value skipped to the next name.

This is the same class of bug as #3660 (falsy 0 collapsed to unset) and #3745 (CLI property lookup reported "" as missing). The helpers now use is not None, matching those PRs and the typed getters already in Config.get_int / get_bool. A missing key still returns the default. A present empty string is a value: int/float/bool raise ValueError (same as any other unparseable string), and get_first_property_value returns "".

Other walrus .get sites (IO, catalog, table locations/metadata) are left for follow-ups.

Are these changes tested?

Yes. Regression tests pass "" for int, float, and bool properties and for get_first_property_value.

• make lint
• make test → 3933 passed, 3 skipped, 1569 deselected

Are there any user-facing changes?

Yes. An explicitly empty int/float/bool property now raises ValueError instead of silently using the default, and get_first_property_value no longer skips an empty string. Changelog label requested.