Handle missing percentile for measure
Handle missing percentile for measure

This allows us to still display meaningful results when we know the
value, cost-savings etc for a measure but not the percentile.

annapowellsmith/openpresc,ebmdatalab/openprescribing,ebmdatalab/openprescribing,ebmdatalab/openprescribing,annapowellsmith/openpresc,annapowellsmith/openpresc,annapowellsmith/openpresc,ebmdatalab/openprescribing