Nav block creates x2 Menus when you have no menus and add a new block to it

[getdave]

x2 menus are being created by the block.

To replicate

• Delete all existing Nav menus
• New Post
• Add Nav block
• Click to add a new Custom Link block
• See UI indicators that menu being created
• Go to http://localhost:8888/wp-admin/edit.php?post_type=wp_navigation
• See x2 menus are created

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If you click the block inserter to add a new block, and select a custom link, two new menus are created:

It would be good to add a test for this!

Originally posted by @scruffian in #47825 (comment)

[scruffian]

If we go ahead with this then this is no longer an issue!