Return none instead of empty string.

Lukasa · Lukasa · commit eeb070135908 · 2015-04-16T07:03:23.000+01:00
diff --git a/hyper/ssl_compat.py b/hyper/ssl_compat.py
@@ -147,7 +147,7 @@ def selected_npn_protocol(self):
         if isinstance(proto, bytes):
             return proto.decode('ascii')
 
-        return proto
+        return proto if proto else None
 
     def getpeercert(self):
         def resolve_alias(alias):
