I18n fix: "" context is not the same as NULL context!

This bug did not appear earlier because the "" default context was actually never used, always NULL context was passed instead. But bpy.app.translations uses "" as default context in its keys (simplifies the hash/comp functions of internal py messages cache)...

So now, Blender prefers NULL (None in python) as default context value, but understands also "" as such.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53951 954f8c5b-7b00-dc11-b283-0030488c597c