Fix #53251: bindtextdomain with null dir doesn't return old value

Apparently, users expect `bindtextdomain` and `bind_textdomain_codeset`
with `null` as second argument to work like thei

Fix #53251: bindtextdomain with null dir doesn't return old value

Apparently, users expect `bindtextdomain` and `bind_textdomain_codeset`
with `null` as second argument to work like their C counterparts,
namely to return the previously set value. Thus, we support that.

Closes GH-6631.

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