hgweb: explicitly tests for None in webutil

Changeset d2878bec55bd 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/webutil.py

@@ -143,7 +143,9 b' class filerevnav(revnav):'
 
 class _siblings(object):
     def __init__(self, siblings=None, hiderev=None):
-        self.siblings = [s for s in siblings or [] if s.node() != nullid]
+        if siblings is None:
+            siblings = []
+        self.siblings = [s for s in siblings if s.node() != nullid]
         if len(self.siblings) == 1 and self.siblings[0].rev() == hiderev:
             self.siblings = []