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

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()
 

testing/test_hookrelay.py

@@ -112,7 +112,7 @@ def hello(self, arg):
 
     pm.register(Plugin1())
     res = pm.hook.hello(arg=3)
-    assert res == None
+    assert res is None
 
 
 def test_firstresult_no_plugin(pm):
@@ -126,4 +126,4 @@ def hello(self, arg):
 
     pm.add_hookspecs(Api)
     res = pm.hook.hello(arg=3)
-    assert res == None
+    assert res is None