[Executors] tryDiagnoseExecutor must survive null witness, when using Backdeploy library

[ktoso]

Description: The recently introduced additional logic to handle missing witnesses was not expecting a null value since in CURRENT stdlib we always have a witness however, in backdeployment we would be missing witnesses and get a null here. We do not test or build backdeployment library in CI, (not even in full tests), which meant this slipped through and was caught in other tests downstream.
Risk: Low, adds missing null check to avoid null pointer dereference.
Reward: Don't crash while building the backdeployment library. There should not be effect on other libraries building though.
Review by: @DougGregor @hborla
Testing:

• CI testing - fake SDK tests were amended to closely replicate how the backdeployment library looks for real (and not just aproximately) -- the adjusted test reproduced the issue and confirms the fix.
• Confirmed manually our previous Xcode reproducer project, still fixed (thanks to [Executors] Do not consider Loc validity in determining to emit error/warning about enqueue(_:) #67957)
  Radar: rdar://114029779

[ktoso]

@swift-ci please smoke test