The influxdb_accumulator filter can generate incorrect data

The filter doesn't check for empty tag values which aren't allowed by the InfluxDB protocol.

For example:

sample,metadata.host=network.node-6.test.domain.local,metadata.name=,metadata.status=DOWN,meter=port,project_id=5f35ff51f94e433ca5bd07dc3aedb05a,resource_id=09cc2e19-e6b1-4ac5-a802-6b9e185e0061,source=openstack type="gauge",value=1.000000,message_id="06084676-06e2-11e6-a7ea-1632710d7aa5",timestamp="2016-04-20 10:24:11.493052",unit="port" 1461147851493100032

The metadata.name tag is empty and eventually the InfluxDB HTTP output plugin will try to send the request forever because the InfluxDB server replies with a 400 response code.