Fix none handling + map fetch rewrite

[gldubc]

Closes #15594 + I noticed some problems with the map fetch code:

1. splitting was done twice using two similar named functions map_split_negative_pairs_key and the take_fun in map_negative
2. two functions were called splitting; renamed the second to map_remove_negative to show it eliminates negatives

I think the duplication was to avoid re-building a list of negatives, but it was doing extra work in the end.

This is an alternative solution to #15593 (which works fine), since removing the no negative "fast path" in map fetch key and testing if the bdd we fetched from is non empty serves the same.

[josevalim]

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