Fix race condition checking if event mutex exists

[vkulov]

Fix issue #50330 a race condition checking if lock already exists.

It's possible for a race condition to occur if we acquire the lock just to check if it's free.

The exists check interferes with the actual lock acquisition at event startup. As a result it's possible to miss an event run.

Imagine following scenario:

1. thread A goes inside the exists method and lock the mutex.

2. All other threads conclude that the event will be run by thread A and skip the event run to avoid overlapping

3. thread A releases the lock by returning from the exists methog

4. thread A proceeds to acquire the lock "for real" but the lock has already been locked by another thread running the exists method.

5. Thread A bails out thinking the event is being run by another thread but in fact it's not.

[crynobone]

Mark as draft since tests are failing