[Bug c/126264] attribute error is ignored with invalid param

[jg at jguk dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126264

--- Comment #1 from Jonny Grant <jg at jguk dot org> ---
warning ("message") is also ignored if the parameter is NULL or nullptr - could
they be addressed together on the same PR?

<source>:4:1: warning: 'warning' attribute ignored [-Wattributes]
    4 | void _fail() __attribute__ ((warning(NULL)));
      | ^~~~

https://godbolt.org/z/x3E7PP5f8

// gcc -std=c20 -Wall -O3 -o error error.c
#include <stddef.h>

void _fail() __attribute__ ((warning(NULL)));

int main()
{
    _fail();
}