pmwebd issues/580: tolerate empty prometheus metrics query
pmwebd issues/580: tolerate empty prometheus metrics query

Previous code could crash on http://localhost:PORT/pmapi/1/metrics
sans the ?target=FOO querystrings.

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