fix: Refactored computed property to avoid force-unwrap

[KristofferHermansen]

Description ✏️

We are encountering fatal errors caused by the force-unwrap used in the observers computed property.
This occurs due to a race condition where asynchronous code attempts to read the count of observers while some of the observers are being deallocated.
This issue was identified during unit tests, where it would occasionally cause random test failures.

Cause 🤔

The current implementation uses the filter function to ensure the map function can unwrap the optional observer property. However, this approach allows observers to be deallocated between the execution of these two functions. If deallocation occurs during this window, the force-unwrap results in a fatal error.

Solution 👨‍💻

The observers property has been refactored to use the compactMap function instead of filter and map. This eliminates the need for a force-unwrap and prevents the error.

Due to the timing-specific nature of the issue, I have not added unit tests to demonstrate the impact. However, after applying this change, the problem no longer occurs in our unit tests.

[KristofferHermansen]

@artman we first observed this problem in our unit tests.
However we have recently started seeing crashes in our production apps.
The crash logs we have, all point back to the Signal.observers.getter as the cause of the crash.
Any chance you have time to take a look at this fix for the problem?