Don't crash on blank PO-Revision-Date / POT-Creation-Date headers

[agu2347]

Fixes #1219.

Some tools (e.g. Poedit) can leave the PO-Revision-Date header blank in a generated .po file instead of omitting it or using the conventional YEAR-MO-DA HO:MI+ZONE placeholder. Parsing such a file crashes with:

ValueError: time data '' does not match format '%Y-%m-%d %H:%M'

because _parse_datetime_header() is called unconditionally with the blank value, as reported in the issue.

Fix: guard both call sites (po-revision-date and, for the same reason, pot-creation-date) to skip parsing when the header value is blank, keeping the previous/default value instead -- the same treatment already given to the YEAR-MO-DA HO:MI+ZONE placeholder value right above it, and to a blank Language header elsewhere in this same method (self._set_locale(value or None)).

Testing: added a regression test that parses a message block with both headers blank and asserts: no exception is raised, creation_date/revision_date fall back to their defaults rather than being corrupted, and no literal None leaks into the round-tripped mime_headers output. I confirmed the test fails with the original ValueError without the fix, and passes with it.

I also verified end-to-end with an actual .po file parse + write round-trip (using an installed release build with CLDR data, since a shallow clone here can't build that data on its own), confirming no None leaks into the output and that normal (non-blank) header values are completely unaffected. Ran the full existing test_catalog.py and test_pofile.py suites: all pass, no regressions.