[Python] Partitioning.dictionaries in case of a subset of fields are dictionary encoded

Follow-up on ARROW-14612, see the discussion at https://github.com/apache/arrow/pull/12530#discussion_r841760449

ARROW-14612 changes the return value of the dictionaries attribute from None to a list in case some of the partitioning schema fields are not dictionary encoded.

But this can result in a non-clear mapping between arrays in Partitioning.dictionaries and fields in Partitioning.schema