Show no-change symbol in KPI module if delta is unchanged.

Also don't attempt to calculate a delta if the previous value is
null/undefined/zero.

This should really be refactored to share code with the delta.js
view used in the multistats module. I'll add a story for this.
Merge pull request #473 from alphagov/bugfix/delta-symbol-for-unchanged-data

Show no-change symbol in KPI module if delta is unchanged.