check_po.pl lists lots of false positives saying that
    msgstr ""
does not have the (h)otkey translated.

This patch whitelists those untranslated strings.

I also tested (by manually "breaking" a translation) that missing
hotkeys still get noticed.

This bug probably exists since forever, therefore I propose this patch
for 2.9..trunk. (OTOH, nobody noticed it, so maybe trunk is enough ;-)

Note: I still get a few false positives for ru.po (no idea why, similar
texts in the other languages don't cause this) - ideas and fixes welcome.