Caching: Don't remove null values from hybrid cache to avoid broken content references repeatedly requiring a database hit (closes #18892)

[Zeegaan]

Fixes #18892

Notes

• Thanks to some investigate work from @enkelmedia, fetching deleted nodes from for example a MultiNodeTree picker, will try to fetch the deleted content, resulting in a Database hit.
• This PR remedies that, by also caching null values.
• A little background on the decision not to cache null values, was that we had no way of deleting nodes with keys that we did not know. Now we can delete all content by tags, so we have a way of removing them again 😁

How to test

• Make content picker datatype which can choose multiple content
• Make a content type using that datatype
• Create 2 different documents, 1 referencing the other, save and publish, then delete the referenced content.
• In your template for the existing content, try getting the value of the multi node picker with @Model.Value("propertytype name here")
• This should no longer result in a database hit (atleast after the first get), as we will cache the null value

[AndyButland]

Have followed the provided test steps and confirm this resolves the issue. Thanks @Zeegaan.