fix: do not force cast KVO values

[kirillzyusko]

📜 Description

Do not use force casting in swift - replaced with optional casting and guard statements.

💡 Motivation and Context

In sentry one of library user seen this:

Thread 0 - (TH_STATE_WAITING)
0  Trusted +0x5746c0        Swift runtime failure: Unexpectedly found nil while unwrapping an Optional value (KeyboardMovementObserver.swift:109:65)
1  Trusted +0x5705cc        KeyboardMovementObserver.observeValue(forKeyPath:of:change:context:) (<compiler-generated>)
2  Foundation +0x99e54      _NSKeyValueNotifyObserver
3  Foundation +0x99cf0      _NSKeyValueDidChange
4  Foundation +0x99b20      -[NSObject(NSKeyValueObservingPrivate) _changeValueForKeys:count:maybeOldValuesDict:maybeNewValuesDict:usingBlock:]
5  Foundation +0x99850      -[NSObject(NSKeyValueObservingPrivate) _changeValueForKey:key:key:usingBlock:]
6  Foundation +0x12cb94     __NSSetPointValueAndNotify
7  UIKitCore +0x615f4       -[UIView(Geometry) _applyISEngineLayoutValuesToBoundsOnly:]
8  UIKitCore +0x60e44       -[UIView(Geometry) _resizeWithOldSuperviewSize:]
9  CoreFoundation +0x353f0  ___NSARRAY_IS_CALLING_OUT_TO_A_BLOCK__

It was very random crash, but still - it's not a good practice to use force casting and as a prof -> it produces a crash in certain cases.

So in this PR I replaced it with more safe operators.

Closes https://github.com/kirillzyusko/react-native-keyboard-controller/issues/93#issuecomment-1946325370

📢 Changelog

iOS

• do not force cast KVO values

🤔 How Has This Been Tested?

There is no way to test whether crashes are gone or not, but I think they should gone (in worst case we will have a new stacktrace).

📝 Checklist

• [x] CI successfully passed
• [x] I added new mocks and corresponding unit-tests if library API was changed

[github-actions[bot]]

📊 Package size report

Current size	Target Size	Difference
109835 bytes	109837 bytes	-2 bytes 📉