When substituting system constant functions with a constant result
the server was not expecting that the function may return NULL.
Fixed by checking for NULL and returning Item_null (in the relevant
collation) if the result of the system constant function was NULL.

mysql-test/r/mysql.result:
  Bug #41437: test case
mysql-test/t/mysql.test:
  Bug #41437: test case.
  Relies on database() returning NULL if no database is
  selected.
sql/item_strfunc.cc:
  Bug #41437: Check for NULL result on evaluating the system
  constant function and return a constant NULL item.