Bug#57820 extractvalue crashes

Problem: ExtractValue and Replace crashed in some cases
due to invalid handling of empty and NULL arguments.

Per file comments:

  @mysql-test/r/ctype_ujis.result
  @mysql-test/r/xml.result
  @mysql-test/t/ctype_ujis.test
  @mysql-test/t/xml.test
  Adding tests

  @sql/item_strfunc.cc
  Make sure Item_func_replace::val_str safely handles empty strings.

  @sql/item_xmlfunc.cc
  set null_value if nodeset_func returned NULL,
  which is possible when the second argument is an
  unset user variable.