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

[ParaskP7]

Parent #2798

This PR adds any missing nullability annotation to TaxonomyXMLRPCClient.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, not so randomly, due to the fact that you've also reviewed #2826, and I wanted you from the Jetpack/WordPress mobile team again 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 fetch term on taxonomy xmlrpc client
2. Add missing n-a to fetch terms on taxonomy xmlrpc client
3. Add missing n-a to push term on taxonomy xmlrpc client
4. Add missing n-a to delete term on taxonomy xmlrpc client
5. Add missing n-a to terms response to terms model method
6. Add missing n-a to term response object to term model method
7. Add missing n-a to term model to content struct method

Nullability Checks List:

1. Remove unnecessary is response null check
2. Guard usages of terms response to terms model method

Warnings Resolutions List:

1. Create missing switch cases for taxonomy xmlrpc client errors
2. Replace value of with long parse long for string reponse

Refactor List:

1. Reformat taxonomy xmlrpc client
2. Replace anonymous classes with lambda

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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:

• Create a new self-hosted WordPress site for XMLRPC testing purposes (jurassic.ninja).
• 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.
⚠️ It seems that this setting is not available for self-hosted WordPress sites.

• 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.
⚠️ It seems that this setting is not available for self-hosted WordPress sites.

• 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.

3. Prepublishing Screens [PrepublishingTagsFragment.kt + PrepublishingCategoriesFragment.kt]

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

• Go to Posts screen and edit any post.
• Click on the UPDATE button to have the bottom sheet appear (top-right).
• Click on the Tags row and add, remove or edit any tag for this post. Click back.
• Click on the Categories row and add, remove or create a new category for this post. Click back.
• Click on the UPDATE NOW button to update this post (bottom).
• Verify that this post's Tags/Categories are updated and that everything works as expected.

[irfano]

I've tested the PR on the FluxC example app and Jetpack (but not on WP to save time). I haven't noticed any issues. 🟢
I only have a comment for you to address.

[ParaskP7]

👋 @irfano and thank you so much for reviewing, testing and commenting on this PR, you rock! 🙇 ❤️

FYI: I left this comment for your that hopefully clears-up this confusion on why I made that change.

[ParaskP7]

Thanks for approving and merging this @irfano ! 🙇 ❤️ 🚀