Make views compatible with Nullable Reference Types

[JasonElkin]

Prerequisites

• I have added steps to test this contribution in the description below

This seeks to resolve the issues discussed in #11289

I appreciate that this PR may need some more work/discussion but I wanted to make a start.

Description

All this really does is add more robust null checking to the views that ship with Umbraco. This will prevent warnings in projects where Nullable Reference Types and Razor View Compilation are turned on.

N.B. Some of the null checks seem unnecessary - but they will be necessary in .net 6. They won't cause any harm in the meantime and will supress issues if devs choose to use/test Umbraco in .net 6.

To test

1. Turn on Nullable Reference Types in and Razor Compilation in the csproj:

<Nullable>enable</Nullable>
...
<RazorCompileOnBuild>true</RazorCompileOnBuild>

2. Build
3. There should be no "Dereference of a possibly null reference." warnings.
4. Use the views, they still work 😅

[umbrabot]

Hi there @JasonElkin, thank you for this contribution! 👍

While we wait for one of the Core Collaborators team to have a look at your work, we wanted to let you know about that we have a checklist for some of the things we will consider during review:

• It's clear what problem this is solving, there's a connected issue or a description of what the changes do and how to test them
• The automated tests all pass (see "Checks" tab on this PR)
• The level of security for this contribution is the same or improved
• The level of performance for this contribution is the same or improved
• Avoids creating breaking changes; note that behavioral changes might also be perceived as breaking
• If this is a new feature, Umbraco HQ provided guidance on the implementation beforehand
• The contribution looks original and the contributor is presumably allowed to share it

Don't worry if you got something wrong. We like to think of a pull request as the start of a conversation, we're happy to provide guidance on improving your contribution.

If you realize that you might want to make some changes then you can do that by adding new commits to the branch you created for this work and pushing new commits. They should then automatically show up as updates to this pull request.

Thanks, from your friendly Umbraco GitHub bot 🤖 🙂

[nul800sebastiaan]

Sorry for the late reply here @JasonElkin but I've finally been able to review this one and it's all good! 👍 Thanks for this one!