Hide appender on BlockPreviews via checking for null component in BlockListAppender

[getdave]

Previously if a null component was passed to the BlockList component it would still render an empty <div className="block-list-appender"> element. This because we didn't check for null before rendering.

This is useful because we sometimes want to disable the rendering of the Block appender UI. For example in block previews.

Required for #16873

How has this been tested?

• Insert quote Block with content
• Switch style of Block
• Inspect the Block Preview of the Style. See no <div className="block-list-appender"> component.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[youknowriad]

Pushed a tweak, merge if it works for you.

[getdave]

Thanks @youknowriad. Using false is much clearer than passing a null Component.