Ensure denominatorMatcher and numeratorMatcher are always defined for completion collections

To avoid `d[this.matchingAttribute].match(this.denominatorMatcher)` (and similar) causing cross-browser issues with `.match(undefined)` returning inconsistent results.
Merge pull request #508 from alphagov/bugfix/completion-numbers-zero-values

Ensure denominatorMatcher and numeratorMatcher are always defined for completion collections