Warn on strtr(["" => "x"])

Previously:
* If only ["" => "x"] was present, the original string was returned
without warning.
* If both ["" => "x"] and at least one more eleme

Warn on strtr(["" => "x"])

Previously:
* If only ["" => "x"] was present, the original string was returned
without warning.
* If both ["" => "x"] and at least one more element was present,
false was returned without warning.

New behavior:
* Ignore "" keys in the replacement array (and perform any remaining
replacement).
* Throw a warning indicating that an empty string replacement has
been ignored.

Closes GH-4792.

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