Description

Fixes #37758.

The useSetting( 'spacing.units' ) can return false, which isn't filterable.

How has this been tested?

1. Activate Twenty Nineteen theme.
2. In the post editor, add Spacer block.
3. Block should be added correctly, and there shouldn't be an error in the console.

Types of changes

Bugfix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).