Ignore blank PO revision dates

[ShipItAndPray]

Summary

• ignore blank PO-Revision-Date header values when loading catalog MIME headers
• preserve the existing placeholder revision date instead of trying to parse an empty string
• add a focused regression test for reading a .po header with a blank revision date

Closes #1219.

Validation

• E1 baseline repro exists: yes
  • On master, pofile.read_po() raises ValueError: time data '' does not match format '%Y-%m-%d %H:%M' when the header contains PO-Revision-Date: with an empty value.
• E2 new regression test fails on baseline: yes
  • In a detached origin/master worktree with only the new test applied:
  • python3 -m pytest tests/messages/test_pofile_read.py -k 'blank_revision_date' -q
  • Result: 1 failed, 31 deselected
• E3 regression test passes after fix: yes
  • python3 -m pytest tests/messages/test_pofile_read.py -k 'blank_revision_date' -q
  • Result: 1 passed, 31 deselected
• E4 nearby targeted behavior passes: yes
  • Direct issue repro now returns a catalog with revision_date == 'YEAR-MO-DA HO:MI+ZONE'
  • python3 -m pytest tests/messages/test_catalog.py -k 'datetime_parsing' -q
  • Result: 1 passed, 43 deselected
• E5 diff stays localized: yes
  • Changes are limited to babel/messages/catalog.py and tests/messages/test_pofile_read.py.

Note: I did not use the full tests/messages/test_pofile_read.py file as a validation gate because this raw source checkout does not have Babel's locale data bundle built, and many unrelated tests in that file fail before reaching this issue's code path.