MR Agg: Enable tests for output mode 'reduce' with a non-trivial reduce function

As part of SERVER-43303, I wasn't able to enable the map_reduce_reduce.js FSM test since the reduce function assumes that each value is an object with a set structure. However, in the translation to agg, we're building a $merge with a pipeline to run when there is a matching document in the target collection. Until we do SERVER-43860 the final reduce will fail since one of the values will be null.