Fix bug in `ZeroUnit` when it is not a dimension

only for dimensions the unit identifier is optional

Change-Id: Ic1396cb3f344550b94a6721d7269733a7fdbbbe6
Reviewed-on: http://gerrit.causes.com/37677
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>