Make empty translation test compatible with translate-toolkit 1.14.0

The API has changed here (due to compatibility with Python 3), so once
new API is available we have to use it.
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/formats.py

@@ -675,8 +675,15 @@ class FileFormat(object):
         if store is None:
             return False
 
-        if cls.monolingual is False and str(store) == b'':
-            return False
+        if cls.monolingual is False:
+            if hasattr(store, 'serialize'):
+                # ttkit API since 1.14.0
+                storebytes = bytes(store)
+            else:
+                # ttkit API 1.13.0 and older
+                storebytes = str(store)
+            if storebytes == b'':
+                return False
 
         return True
 

weblate/trans/tests/test_files.py

@@ -302,7 +302,6 @@ class ImportMoPoTest(ImportTest):
     test_file = TEST_MO
 
     def create_subproject(self):
-        # Needs to create PO file to have language pack option
         return self.create_po()