Allow removal of field values for previously created block documents

[desertaxle]

First check

• [X] I added a descriptive title to this issue.
• [X] I used the GitHub search to find a similar request and didn't find it.
• [X] I searched the Prefect documentation for this feature.

Prefect Version

2.x

Describe the current behavior

There is a PATCH API endpoint for block documents that allows the update of values within block documents, but it doesn't handle values for optional fields being removed from the block document.

Describe the proposed behavior

The API should allow values for block document fields to be removed from existing block documents.

Example Use

No response

Additional context

No response