Make preference node renderers more robust against null values

[colin-grant-work]

What it does

A number of preferences use null as a default even if that is not among their declared values. As a consequence, it is possible for a preference renderer to encounter a null value when another value type is expected.

How to test

1. Start the application.
2. Open the preferences UI
3. On master, you will see a message about inability to read toString of null and the layout may fail to be restored.
4. With this code, you will see no message about inability to read toString of null and the layout should successfully be restored. The preference html.format.maxPreserveNewLines should show an empty input box, unless you have set a numerical value for it.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

I confirmed that the changes work well 👍

The error on master is no longer present, and input fields such as html.format.maxPreserveNewLines now properly render by default (null value).