fix: do not substitute delegate back if it's nil

[kirillzyusko]

📜 Description

Fixes a problem when all events such as onFocus/onBlur/etc. will stop working after keyboard dismissal.

💡 Motivation and Context

It looks like a delegate in original TextInput can be somehow substituted and we are loosing a pointer to original delegate. If on keyboard-hide event we'll substitute delegate with nil - we will make all events, such as onFocus, onBlur etc. not coming to JS.

So the most obvious decision is to check, if we actually have a delegate and do a substitution only in this case.

Of course a rhetorical question arises - if we had original delegate, then injected our delegate, then it was substituted with a new one -> turns out that our events will not come, right? Right. But:

• I don't know how it's possible - I checked RN sources and I don't see a way how delegate can be injected after component creation;
• for now it's still safer to miss events in keyboard-controller rather than disable all events at framework level;
• I can not reliably reproduce this in empty project and the issue is reproducible on relatively old devices (iO 15.2).

So yeah, the problem is somewhere deeper, but I don't have an access to the code so can't debug what exactly happens there. So for now it'll be safer not to ruin the RN framework and merge this fix 🙃

Closes #456

📢 Changelog

iOS

• check previous delegate presence when substitute delegate back;

🤔 How Has This Been Tested?

Tested manually on iOS 15.2

📸 Screenshots (if appropriate):

Before	After
before.mp4	after.mp4

📝 Checklist

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

[github-actions]

📊 Package size report

Current size	Target Size	Difference
157531 bytes	157518 bytes	13 bytes 📈