attempt to prevent pebble to only read SGV values

Try to avoid reporting null for pebble by only considering SGV
records.

Many thanks to @elute for raising flags on this.
fix mmol panning to show mmol

Fixes a bug where panning would show "HIGH".
Merge pull request #200 from nightscout/wip/fix-null-pebble

attempt to prevent pebble to only read SGV values