Preserve blank query values when combining URLs

[jaredrainsha]

URL query parameters with empty values are silently dropped when parsing existing paths: url("https://example.com/path?a=&b=1") returns only b=1, while an empty string supplied through keyword arguments is kept.

Preserve blank values when parsing each path's query string. Add six regression/compatibility cases covering explicit and bare empty values, repeated keys, an empty key, multiple paths, and keyword parameters. The tests compare complete URLs so parsing the expected result cannot hide a missing parameter.

Validation:

• The same six cases on unchanged develop: 5 failed, 1 passed; with the fix: 6 passed.
• tox run -e py310,py311,py312,py313,py314: all five environments passed inv ci, including package/docs builds, ruff, mypy, generated chaining-type consistency, and 2,482 tests/doctests per version with 100% coverage.
• The initial run exposed an existing unquoted temporary-path command when the checkout path contained a space. The complete matrix was rerun successfully with the identical patch in a path without spaces; no checks were changed or skipped.

AI assistance: Codex assisted with reproduction, implementation, regression tests, validation, diff review, and PR preparation.

[coveralls]

coverage: 100.0%. remained the same — jaredrainsha:fix/url-blank-query-values into dgilland:develop