Fix pylint warnings inconsistent-return-statements

Add consistent return to all functions and methods that are covered by
tox -e pylint[23]. I haven't checked if return None is always a good
idea or if we should rather raise an error.

See: https://pagure.io/freeipa/issue/7326
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

ipalib/frontend.py

@@ -1005,6 +1005,8 @@ class Command(HasParam):
     def get_summary_default(self, output):
         if self.msg_summary:
             return self.msg_summary % output
+        else:
+            return None
 
     def log_messages(self, output):
         logger_functions = dict(
@@ -1035,7 +1037,7 @@ class Command(HasParam):
         Subclasses can override this method, if custom output is needed.
         """
         if not isinstance(output, dict):
-            return
+            return None
 
         rv = 0