Avoid calling None in call_historic

Thanks to @rmfitzpatrick for this fix!

Author: Tyler Goodlet

pluggy/hooks.py

@@ -265,6 +265,8 @@ def call_historic(self, proc=None, kwargs=None):
         self._call_history.append((kwargs or {}, proc))
         # historizing hooks don't return results
         res = self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
+        if proc is None:
+            return
         for x in res or []:
             proc(x)