pmproxy: return {} if no label values are found, to be consistent with regular response
pmproxy: return {} if no label values are found, to be consistent with regular response

Regular response is: { labelName: [labelValues...] }

Returning an empty array if no label value is found breaks APIs which
deserialize the JSON with a predefined expected schema.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp