Add fallback styles for dropdown block when css vars are not populated

[talldan]

Note - this PR merges into #42890 rather than trunk.

Proposed changes:

It was noted in #43096 (review) that often when dropdown field is shown in a preview, it's missing styles.

One of the reasons this happens is explained in #41418.

I noticed there's also another contributing factor that explains why this happens for the dropdown block but not the multiline/textarea block.

The latter has fallback values set in its CSS, while the former doesn't.

In this PR I'm also setting fallbacks for the dropdown, and it seems to mostly solve the preview problems, at least in situations where the theme isn't changing the styles of the form much.

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Testing instructions:

1. Create a new post
2. Open the inserter
3. Search for 'Dropdown'
4. Hover the dropdown field block to see the preview

Expected: The block should have proper border styles and padding.

Screenshots

Before

After

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Code Coverage Summary

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[aaronrobertshaw]

Loving all the fixes, keep 'em coming 🚀

✅ Works for me for both field and form variation previews
✅ Couldn't break the default styling regardless of method of adding field, form variation, undo/redo, pasting etc.

LGTM 👍