Fix to BLF_pgettext, which was returning "" when passed NULL msgid... Never noticed any problem, but it looks it could produce strange UI issues (like deactivated buttons), and it was not consistent anyway!

Thanks to S. Lockal for spotting the issue and providing a patch.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55117 954f8c5b-7b00-dc11-b283-0030488c597c