Preferences: Render node only if present in schema

[colin-grant-work]

What it does

Fixes #9241 by not attempting to render a node in the PreferencesEditorWidget if we can't retrieve any information about that preference from the PreferenceService.

How to test

1. Install the plugins described TypeError: Cannot read property 'defaultValue' of undefined when uninstalling a VS Code extension #9241.
2. Uninstall them using the Extensions widget.
3. You should see no errors in the console about attempts to access fields on an undefined variable.

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

[colin-grant-work]

@kittaakos, as I say in the description, the first commit should be enough to fix the error you observed. The second commit has more to do with cleaning up some pretty funky code from the implementation of the preferences widget. It's a lot of changes though, so happy to go with the narrower bugfix, if that seems more appropriate.

I decided just to go with the minimal fix, for now.

[vince-fugnitto]

I confirmed that it fixes the error reported in #9241.
I no longer see the following error, and the preferences-view is correctly updated on uninstall:

single-preference-wrapper.tsx:79 Uncaught TypeError: Cannot read property 'defaultValue' of undefined
    at SinglePreferenceWrapper.push.../../packages/preferences/lib/browser/views/components/single-preference-wrapper.js.SinglePreferenceWrapper.render (single-preference-wrapper.tsx:79)
    at finishClassComponent (react-dom.development.js:17160)
    at updateClassComponent (react-dom.development.js:17110)
    at beginWork (react-dom.development.js:18620)
    at HTMLUnknownElement.callCallback (react-dom.development.js:188)
    at Object.invokeGuardedCallbackDev (react-dom.development.js:237)
    at invokeGuardedCallback (react-dom.development.js:292)
    at beginWork$1 (react-dom.development.js:23203)
    at performUnitOfWork (react-dom.development.js:22157)
    at workLoopSync (react-dom.development.js:22130)