Handle firstresult hooks that return None

Only return the first result when at least one result has been returned
by underlying hook implementations.

Fixes #68

Author: Tyler Goodlet

pluggy/callers.py

@@ -107,7 +107,8 @@ def execute(self):
                     pass
 
             if firstresult:
-                return outcome.get_result()[0]
+                result = outcome.get_result()
+                return result[0] if result else None
 
             return outcome.get_result()