[Nullability Annotations to Java Classes] Add Missing Nullability Annotations to TaxonomyRestClient (safe)

[ParaskP7]

Parent #2798

This PR adds any missing nullability annotation to TaxonomyRestClient.java, all its related response classes and anything in between (ie. TaxonomyStore). See response classes below:

• TermWPComRestResponse.java

FYI.1: This change is safe, meaning that there shouldn't be any (major) compile-time changes associated with this change. Having said that, testing is highly recommended since the changes in this PR can affect one or more clients (ie. JP/WPAndroid, WCAndroid, etc)

FYI.2: An analysis on TaxonomyStore and its usages with our main clients suggests that this store is only being utilizing JP/WPAndroid. AFAIA WCAndroid is not using that store and as such can be safely ignored from testing.

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PS: @irfano I added you as the main reviewer, randomly so, since I just wanted someone from the Jetpack/WordPress mobile team to be aware of and sign-off on that change for JP/WPAndroid. Feel free to merge this PR directly yourself if you deem so.

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Nullability Annotation List:

1. Add missing n-a to instantiate term model methods
2. Add missing n-a to remote term payload and children
3. Add missing n-a to fetch term on taxonomy rest client
4. Add missing n-a to fetch terms on taxonomy rest client
5. Add missing n-a to push term on taxonomy rest client
6. Add missing n-a to delete term on taxonomy rest client
7. Add missing n-a to term response to term model method on trc
8. Add missing n-a to term model to params method on trc
9. Add missing n-a to term wp com rest response fields

Nullability Checks List:

1. Guard usages of instantiate term model methods

Warnings Resolutions List:

1. Change type of request to include the missing generic type
2. Make inner classes static on term wp com rest response

Refactor List:

1. Reformat taxonomy rest client
2. Replace anonymous classes with lambda
3. Remove unused meta field from term wp com rest response

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Associated Clients

It is highly recommending that this change is being tested on the below associated clients:

• JP/WPAndroid

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To Test:

• Smoke test the FluxC Example app via the TAXONOMIES screen. Verify everything is working as expected.
• In addition to the above, using local-builds.gradle on JP/WPAndroid, smoke test any TaxonomyStore related functionality on both, the WordPress and Jetpack apps, and see if everything is working as expected. For a couple of examples, you can expand and follow the inner and more explicit test steps within:

1. Categories Settings Screen [CategoriesListFragment.kt]

ℹ️ This test applies to both, the Jetpack and WordPress app.

• Go to Menu sub-tab on the initial My Site screen and click on Site Settings under the
  Manage section.
• Find the Categories under the Writing section and click on it.
• Verify that the Categories screen is displayed and that everything works as expected.

2. Tags Settings Screen [SiteSettingsTagListActivity.kt]

ℹ️ This test applies to both, the Jetpack and WordPress app.

• Go to Menu sub-tab on the initial My Site screen and click on Site Settings under the
  Manage section.
• Find the Tags under the Writing section and click on it.
• Verify that the Tags screen is displayed and that everything works as expected.

[irfano]

Thank you for well structured commits and helpful PR descriptions. LGTM! 👍🏻
I've also tested FluxC example app and JP app and couldn't discover any issue. 🟢
I've added two questions just for my curious, not a blocker but haven't merged the PR just in case. You can merge it after answering my questions.