Fix empty time ranges from 500 exceptions

[gmunoz]

Fixes #144

A divide by zero scenario when the time range is empty.

Tested an empty result set using:

curl -k -u dev:dev -i --get \
    'https://192.168.121.189:4433/api/metrics/nodes_cores/summary' \
    --data-urlencode 'serviceName=local-dev' \
    --data-urlencode 'startTime=2030-01-01T00:00:00Z' \
    --data-urlencode 'endTime=2030-01-01T01:00:00Z' \
    --data-urlencode 'ignoreZeros=true'

The zero-duration case requires an existing profiler-process record with ts == last_seen, or an active process queried with identical startTime and endTime. In that scenario a curl such as the following can exercise the path:

t="$(date -u -d '1 minute ago' '+%Y-%m-%dT%H:%M:%SZ')"; \
  curl -k -u dev:dev -i --get \
    'https://192.168.121.189:4433/api/metrics/nodes_cores/summary' \
    --data-urlencode 'serviceName=local-dev' \
    --data-urlencode "startTime=$t" \
    --data-urlencode "endTime=$t" \
    --data-urlencode 'ignoreZeros=true'