Ignore blank PO-Revision-Date/POT-Creation-Date header values

[agu2347]

Summary

pybabel update/compile crashes with an unhandled ValueError when a .po file has a blank PO-Revision-Date (or POT-Creation-Date) header value, as generated by some tools (e.g. Poedit) instead of the conventional YEAR-MO-DA HO:MI+ZONE placeholder or omitting the header entirely.

File ".../babel/messages/catalog.py", line 109, in _parse_datetime_header
    dt = datetime.datetime.strptime(match.group('datetime'), '%Y-%m-%d %H:%M')
ValueError: time data '' does not match format '%Y-%m-%d %H:%M'

Root cause

_parse_datetime_header() unconditionally passes its input to datetime.strptime(). The po-revision-date call site already special-cases the literal 'YEAR-MO-DA HO:MI+ZONE' placeholder to avoid parsing it, but doesn't handle a blank value, and the pot-creation-date call site has no such guard at all.

Fix

_parse_datetime_header() now returns None for a blank (or whitespace-only) value instead of raising. Both call sites in Catalog._set_mime_headers() only assign creation_date/revision_date when the parse actually returns a value, so a blank header now falls back to the existing defaults instead of crashing -- the same behavior the 'YEAR-MO-DA HO:MI+ZONE' placeholder already gets.

Testing

• Added test_datetime_parsing_blank_value (unit test for _parse_datetime_header) and test_set_mime_headers_ignores_blank_dates (integration test through Catalog._set_mime_headers) to tests/messages/test_catalog.py.
• tests/messages/test_catalog.py: 46 passed.
• tests/messages/ (full, with CLDR data built via scripts/download_import_cldr.py): 383 passed, 1 skipped (the 3 apparent failures without CLDR/cwd setup are pre-existing and unrelated -- they pass once run from the repo root).
• Reverting only babel/messages/catalog.py (keeping the new tests) reproduces the exact reported error: ValueError: time data '' does not match format '%Y-%m-%d %H:%M'.
• ruff check clean on both changed files.

Fixes #1219