Fix file_matches to never return None

Should fix https://github.com/kivy/buildozer/issues/431#issuecomment-264732499 , as the deleted code just makes the function return None when presumably an empty list is fine and it should just continue.

Making a PR rather than a straight change in case I'm missing something about what's intended here.

buildozer/__init__.py

@@ -582,8 +582,6 @@ class Buildozer(object):
         result = []
         for pattern in patterns:
             matches = glob(expanduser(pattern.strip()))
-            if not matches:
-                return
             result.extend(matches)
         return result