Fix nullability of return types that can be non-null

[IvovdBruggen]

This fixes #15926

Changes

Fixes nullability of methods exposed in various Umbraco extensions.

[github-actions]

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[IvovdBruggen]

Realising now that this is a very small pull request, does it sound good that I also fix all the other nullability return warnings? Excluding obsolete methods that is.

[georgebid]

Hey @IvovdBruggen! Thank you for your PR to fix #15926, someone on the team will review this soon 😄 and in terms of creating a new issue, it's up to you really, you could either create a new one or add on to 15926, and then you could update your PR too 😄 And I agree, probably best to leave anything that is obsolete.

[IvovdBruggen]

@georgebid Great! I have updated the issue and PR and will commit the other fixes!

[IvovdBruggen]

Any progress on this perhaps? Came by the same issue today