assure that consolidation.Consolidate always gets a non-None consolidator

[Dieterbe]

fix #1814
Note that "special functions" resetting the consolidator is part of the current design, as described in
https://github.com/grafana/metrictank/blob/master/devdocs/expr.md, section about consolidateBy

• when returning data back to the user via a json response and whatnot, we can consolidate down using the method requested by the user (or average, if not specified). Likewise here, when the setting encounters a special* function while traveling up to the root, the consolidation value is reset to the default (average)

arguably, this design needs to be revised completely as proposed in #1490, or until we do, special functions should set the consolidator to average rather than None, which would be more robust, and depending on how you interpret the spec, implement it more accurately, rather than having to add these checks after functions have run.

But for now, this does the job.

[fkaleo]

perfect