hgweb: explicitly tests for None

Changeset 7dafa8d0e006 removed the mutable default value, but did not explicitly
tested for None. Such implicit testing can introduce semantic and performance
issue. We move to an explicit testing for None as recommended by PEP8:

https://www.python.org/dev/peps/pep-0008/#programming-recommendations

mercurial/hgweb/common.py

@@ -96,7 +96,9 b' class ErrorResponse(Exception):'
             message = _statusmessage(code)
         Exception.__init__(self, message)
         self.code = code
-        self.headers = headers or []
+        if headers is None:
+            headers = []
+        self.headers = headers
 
 class continuereader(object):
     def __init__(self, f, write):