[2.13..3.1] Ignore ´//null-` peers in signal and ptrace events
Ideally we'd update them to the chosen exec target - but until this is implemented, it doesn't make sense to ask about adding a //null-* peer to a profile.
This commit is a manual backport of 41df2ca3 / !1090 (merged) (with indentation changed to match the 3.1 branch)
I propose this patch for 2.13..3.1