Page Data required fields + preview refinement

Overview

Follow these steps

• Edit an entity in XB using the page data form. The entity title field must be required (and should be already)
• Start with their being something in the title field, then change it to empty
• The preview will show the last non-empty value of the field instead. So if you had a title "HELLO" and hit the delete key 5 times to remove the letters, the input is empty, but the preview has H

This isn't an issue with the component props form. With that form, the preview still updates when there are empty required string inputs, and the value displayed is an empty string.

There's a wish for the page data form to function similarly, but doing so requires a different implementation.
Component prop forms validate with JSON schema, which allows empty strings as a required value (this is still enforced when attempting to publish changes, though - empty required string fields will prevent publishing).

For page data, the reason the "last non-empty value" is retained in the preview is we prevent the empty text field from even being sent as the form is programmatically submitted during this stage and would error due to the invalid empty value. Ideally we'll find a way around this to replicate the component inputs behavior, but if this proves convoluted we should be open to other solutions.

Proposed resolution

User interface changes