Handling argc==0 in the kernel

setuid programs already need to defend against argv[0] == "" (or any other actual string, for that matter) because they must not trust the caller. POSIX explicitly specifies that the caller can set argv[0] to whatever it wants, and passing an invalid or incorrect argv[0] has been well understood for a long time as A Thing That Can Be Done (even though it's probably a bad idea in most cases). OTOH, the "there is no argv[0]" case is much less obvious and (I think) is not supported at all on some platforms, so it should not be too surprising that at least one setuid program failed to check for it.

So, from a security perspective, changing "the string doesn't exist" to "the string is empty" is the most straightforward way to close the security hole, since the callee is very likely already checking for an empty string (to the extent that it cares about argv[0] at all).