Problem: SELECT ... REGEXP BINARY NULL may lead to server crash/hang.

Fix: properly handle NULL regular expressions.


mysql-test/r/func_regexp.result:
  Fix for bug #39021: SELECT REGEXP BINARY NULL never returns
    - test result.
mysql-test/t/func_regexp.test:
  Fix for bug #39021: SELECT REGEXP BINARY NULL never returns
    - test case.
sql/item_cmpfunc.cc:
  Fix for bug #39021: SELECT REGEXP BINARY NULL never returns
    - checking regular expressions' null_value
  we tested it without a val_xxx() call before, which is wrong.
  Now Item_func_regex::regcomp() returns -1 in the case
  and allows to handle NULL expessions properly.
sql/item_cmpfunc.h:
  Fix for bug #39021: SELECT REGEXP BINARY NULL never returns
    - checking regular expressions' null_value
  we tested it without a val_xxx() call before, which is wrong.
  Now Item_func_regex::regcomp() returns -1 in the case
  and allows to handle NULL expessions properly.