Consider allowing transformer to run with no changesets

[nick4598]

One such example where this might be useful is a filter transform. A user may wish to change the filter criteria, in which the transformer needs to be rerun to process this new filter.

Currently services team expects to always process changesets in addition to changing the filter criteria, but it may be realistic to change the filter criteria and have no changesets to process. There are currently places where transformer exits early if there are no changesets.

One such place is if transformer._sourceDbChanges is undefined (which it currently is if there are no changesets). This would have to be changed.
if (csFileProps === undefined) return undefined;

needs more thought / investigation