phabricator: avoid passing None to pycompat.fsdecode

Differential Revision: https://phab.mercurial-scm.org/D8525

hgext/phabricator.py

@@ -238,8 +238,8 b' def vcrcommand(name, flags, spec, helpca'
 
     def decorate(fn):
         def inner(*args, **kwargs):
-            cassette = pycompat.fsdecode(kwargs.pop('test_vcr', None))
-            if cassette:
+            if kwargs.get('test_vcr'):
+                cassette = pycompat.fsdecode(kwargs.pop('test_vcr'))
                 import hgdemandimport
 
                 with hgdemandimport.deactivated():