selinux: accept the fact that getxyzcon() can return success and NULL

selinux: accept the fact that getxyzcon() can return success and NULL

Inspired by #25664: let's check explicitly for NULL everywhere we do one
of those getXYZcon() calls.

We usually turn this into EOPNOTSUPP, as when selinux is off (which is
supposed to be the only case this can happen according to selinux docs)
we otherwise return EOPNOTSUPP in that case.

Note that in most cases we have an explicit mac_selinux_use() call
beforehand, hence this should mostly not be triggerable codepaths.