Fix for uiButGetStrInfo: We need a string value here, return BPY_I18NCONTEXT_DEFAULT_BPY when using default NULL label's context.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54808 954f8c5b-7b00-dc11-b283-0030488c597c