Honor print-charset-text-property value of nil (Bug#31376)
* src/print.c (print_check_string_charset_prop): Move check
for nil Vprint_charset_text_property from here...
(print_prune_string_charset): ... to here.
(syms_of_print) <print-charset-text-property>: Clarify that any
non-boolean values are treated the same as `default'.
* doc/lispref/streams.texi (Output Variables): Add
print-prune-string-charset.
* test/src/print-tests.el (print-charset-text-property-nil)
(print-charset-text-property-default)
(print-charset-text-property-t): New tests.
(print-tests--prints-with-charset-p): New helper function.