Return zero values fr completion when provided with zero input

Completion collection was incorrectly parsing 0 as "no data", which caused errors when datasets contain actual zero values.
Merge pull request #507 from alphagov/bugfix/completion-numbers-zero-values

Return zero values for completion when provided with zero input
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