UGC: Fix an edge case where the app could crash when blocking a post author

[salimbraksa]

Fixes #20387

This PR resolves an issue where the app could crash when blocking a Post Author.

Root Cause

1. Part of the User Blocking implementation is to read the post.authorID to check whether it's blocked or not.
2. The type of post.authorID is NSNumber!
3. The code accessed post.authorID without safely unwrapping it. ( I thought authorID property would never be nil )
4. There is a case where post.authorID could be nil I don't know how to reproduce that case, though.

Solution
Safely unwrap that property before using it.

guard let authorID = post.authorID else {
    return true
}
// Use authorID

Test Instructions

I don't know how to reproduce a case where post.authorID is nil but I think we should test User Blocking feature for regression.

Since the code changes are minor and are unlikely to cause a regression issue, feel free to test only 1 one of the following use cases:

User Blocking Test Instructions

Regression Notes

1. Potential unintended areas of impact
   Smoke test User Blocking feature.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manual testing.

3. What automated tests I added (or what prevented me from doing so)
   None.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 4 days Please, make sure to get it merged by then or assign it to a later expiring milestone
⚠️	This PR contains changes to RELEASE_NOTES.txt. Note that these changes won't affect the final version of the release notes as this version is in code freeze. Please, get in touch with a release manager if you want to update the final release notes.

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[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr20421-3450b7d
	Version	22.0
	Bundle ID	org.wordpress.alpha
	Commit	3450b7d
	App Center Build	WPiOS - One-Offs #5340

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr20421-3450b7d
	Version	22.0
	Bundle ID	com.jetpack.alpha
	Commit	3450b7d
	App Center Build	jetpack-installable-builds #4367

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[alpavanoglu]

LGTM

[mokagio]

It would be good to look into a way to write a unit test for this.