[easy] Set.cardinal > 0 -> not Set.is_empty
[easy] Set.cardinal > 0 -> not Set.is_empty

Summary:
`Set.cardinal` is O(n), `Set.is_empty` is O(1):

https://github.com/ocaml/ocaml/blob/ea55b8b59040b6c5a56d8ddce23e89db87d185ec/stdlib/set.ml#L413-L415

So, if you only care if the cardinality is > 0, then use `not is_empty`.

Reviewed By: dsainati1

Differential Revision: D18067425

fbshipit-source-id: ebdcf667f1e3e1747acac7c9a2d7342a7ab6be55

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