app: allows for empty string as language.

Language detection was not working fine on Windows when the language was
an empty string rather than NULL.

app/language.c

@@ -52,7 +52,7 @@ language_init (const gchar *language)
     return actual_language;
 
 #ifdef G_OS_WIN32
-  if (! language                       &&
+  if ((! language || strlen (language) == 0) &&
       g_getenv ("LANG")        == NULL       &&
       g_getenv ("LC_MESSAGES") == NULL       &&
       g_getenv ("LC_ALL")      == NULL       &&