Ensure Nullability annotations for ListExtractor.java fields

[HatakeKakashri]

Checklist

• I am able to reproduce the bug with the latest version given here: CLICK THIS LINK.
• I am aware that this issue is being opened for the NewPipe Extractor, NOT the app, and my bug report will be dismissed otherwise.
• I made sure that there are no existing issues - open or closed - which I could contribute my information to.
• I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• This issue contains only one bug.
• I have read and understood the contribution guidelines.

Affected version

0.24.6

Steps to reproduce the bug

The ListExtractor.java currently do not expose the nullable fields using @Nullable annotations. Some use-sites might be able to get away with using these fields as non-null and lead to crashes, like TeamNewPipe/NewPipe#12401

This could have been caught early on if the ListExtractor function correctly indicated the nullable behavior of the returned value. Also, given the app is being rewritten in Kotlin, so the correct annotation would enforce the nullability handling at use-site.

Expected behavior

The ListExtractor shall correctly indicate the Nullable annotation for it's fields

Actual behavior

No response

Screenshots/Screen recordings

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Logs

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Additional information

No response