gsettings: Don't translate ""

The empty msgid is traditionally used to store po file metadata,
so calling gettext with an empty msgid is not the right thing
to do.

https://bugzilla.gnome.org/show_bug.cgi?id=756214

gio/gsettingsschema.c

@@ -627,7 +627,7 @@ end_element (GMarkupParseContext *context,
 
           normalised = normalise_whitespace (info->string->str);
 
-          if (gettext_domain)
+          if (gettext_domain && normalised[0])
             {
               gchar *translated;