Avoid squasing translations without any files

This can happen with multi file backend where no files are created yet.

Issue #1525
Fixes HOSTED-2H

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/addons/git.py

@@ -72,6 +72,8 @@ def squash_language(self, component, repository):
 
         messages = {}
         for code, filenames in languages.items():
+            if not filenames:
+                continue
             messages[code] = repository.execute([
                 'log', '--format=%B', '{}..HEAD'.format(remote), '--'
             ] + filenames)