tests_functional: Remove invalid codelist warning for null

This is now failing due to a quirk in the way in which null is
interpreted when not in an embedded codelist. The lack of message here
is consistent with other codelists. This behaviour changed as a result
of using a local version of the grant standard and downloaded codelist
when "resolving" the standard.

ThreeSixtyGiving/standard#379

cove/cove_360/tests_functional.py

@@ -109,7 +109,6 @@ def server_url(request, live_server):
     ('nulls.json', [
         'is not a JSON array',
         'Date is not in the correct format',
-        'Invalid code found in countryCode',
         'is not a number',
         'is not a string',
     ], True),