mq: explicitly tests for None

Changeset fd3d8eb7f545 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

hgext/mq.py

@@ -722,7 +722,8 b' class queue(object):'
 
     def printdiff(self, repo, diffopts, node1, node2=None, files=None,
                   fp=None, changes=None, opts=None):
-        opts = opts or {}
+        if opts is None:
+            opts = {}
         stat = opts.get('stat')
         m = scmutil.match(repo[node1], files, opts)
         cmdutil.diffordiffstat(self.ui, repo, diffopts, node1, node2,  m,