rebase: explicitly tests for None

Changeset 361bccce566a removed the mutable default value, but did not explicitly
tested for None. Such implicit checking can introduce semantic and performance
issue. We move to an explicit check for None as recommended by PEP8:

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

hgext/rebase.py

@@ -725,7 +725,8 b' def _definesets(ui, repo, destf=None, sr'
                 destspace=None):
     """use revisions argument to define destination and rebase set
     """
-    revf = revf or []
+    if revf is None:
+        revf = []
 
     # destspace is here to work around issues with `hg pull --rebase` see
     # issue5214 for details