[Property Editor] The value for parameter 'X' cannot be null

Our analytics show that a number of the failed apply edit requests in the Property Editor are failing with:

```
The value for parameter 'X' cannot be null
```

It's possible we are incorrectly allowing some properties to be nullable that are not actually nullable.

Start in the Property Editor flow that submits apply edit requests and trace how nullable properties are accepted before the request is sent. Reproduce or inspect the failures described in the issue, identify which properties can produce a null value for parameter 'X', and verify that invalid requests are no longer allowed.

Written by the indexing model from the issue text.