Convert empty filter dictionary to None

The filter parameter to stack index over RPC is is being set
to an empty dictionary.  This occurs because  get_allowed_params
returns an empty dictionary in a majority of cases.  This makes a
mess of the test cases in later oslo.messaging testing.  It also
is less effecient than just not serializing the dictionary over RPC.

Change-Id: Ibfcf950243b633cf703ad05263e3ba9e11dfe5bc
blueprint: oslo-messaging
Merge "Convert empty filter dictionary to None"