Issue is that the real default context is NULL, however, in python and RNA, this value can't be used easily. So we use a specific string instead ("*"), defined as BLF_I18NCONTEXT_DEFAULT_BPYRNA.
From now on, all bpy/rna code should only use the BLF_I18NCONTEXT_DEFAULT_BPYRNA value, while all "usual" C code should use the BLF_I18NCONTEXT_DEFAULT value (BLF_pgettext is still able to "understand" both, anyway).
Also added BLF_is_default_context helper func, so that we can keep that check in a single place!
Finally, we should no need anymore to understand the void string "" as default context too - two values for a same thing are more than enough!