Return false when dotenv contains only valueless declarations

[srijan2607]

load_dotenv() returns True for content containing only bare declarations such as EXAMPLE_KEY, although those declarations are ignored and cannot set environment variables.

Return False when all parsed values are None, matching the existing empty-configuration result. Empty-string assignments remain valid, and valued assignments skipped by override=False retain their existing result. Parsing, interpolation and environment contents are unchanged.

Add 22 regression cases covering file/stream inputs, both override modes, existing environment values, bare-only declarations, empty assignments and mixed content.

Testing on Python 3.13.13:

• Original implementation with the new tests: 16 failed, 6 passed (expected negative control).
• Patched regression tests: 22 passed.
• Full suite: 306 passed, 1 skipped (optional IPython unavailable).
• Ruff lint/format checks and git diff --check passed.

Kept as a draft pending CI verification; local results are not a CI pass. Windows and other Python versions have not been tested locally.

[srijan2607]

All nine CI test jobs are passing. Could this be reviewed when convenient?