[1.7.x] Fixed #22773 -- Forced templatize() to return unicode

Backport of daaeb84158 from master.

django/utils/translation/trans_real.py

@@ -544,7 +544,7 @@ def templatize(src, origin=None):
     from django.template import (Lexer, TOKEN_TEXT, TOKEN_VAR, TOKEN_BLOCK,
             TOKEN_COMMENT, TRANSLATOR_COMMENT_MARK)
     src = force_text(src, settings.FILE_CHARSET)
-    out = StringIO()
+    out = StringIO('')
     message_context = None
     intrans = False
     inplural = False