Helper method for Item_func_json_* methods. Check whether an argument can be converted to a utf8mb4 string. @param[in] arg_item An argument Item @param[out] value Where to materialize the arg_item's string value @param[out] utf8_res Buffer for use by ensure_utf8mb4. @param[in] func_name Name of the user-invoked JSON_ function @param[out] safep String pointer after any
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