Avoid mutable default args in _test_populate_filter_props

Using [] as an argument means that any changes to that argument
will stick across multiple calls to the method.  In this case,
this is surprising behavior that may lead to confusion as we
reuse this method in more tests.  Instead, set it to None, which
is non-mutable, and later assign it to [] inside method scope.

Change-Id: I9cc07b8917ad096c5efea9282bae08c41d15f1cf
Merge "Avoid mutable default args in _test_populate_filter_props"