Bug #41437: Value stored in 'case' lacks charset, causes segfault

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

@@ -186,4 +186,6 @@ delimiter
 2
 2
 2
+@z:='1'	@z=database()
+1	NULL
 End of 5.0 tests