Add filter to opt out of automattic Nav fallback behaviour

[getdave]

With the merge of #48698 a fallback Navigation Menu will now automatically be created when an empty (i.e. no ref attribute) Navigation block is present

• in the Editor
• on the front of the site

Given this, should the feature be "opt in"? To do this we would need to:

• add a Boolean filter defaulting to true.
• conditionalise the fallback routine to return empty blocks if the filter is falsey.

I think it would be ok to allow the editor REST call to occur, but if it returns empty then the ref would not be set to the fallback ID so no fallback would be set resulting in an empty menu.

Curious to hear thoughts before we proceed - @scruffian and @draganescu.

[scruffian]

Has this come from a user request? Maybe it's worth seeing if there's a need before we add another filter to core?

[getdave]

I was basing this on @hellofromtonya's previous advice/mantra that if functionality is automatic or new, it should be possible to opt out of it. We don't currently have any specific user request.

[draganescu]

I think the only thing the opt out would do is avoid one single post being created. Doesn't look like it's worth another filter. I do think it's good practice to enable opting out of things, and it is in accordance with the WordPress way but sometimes it just feels like it's not worth the hassle. There are the important filters in place - those to modify what the fallback does - so the one that just disables the saving seems a bit pointless?

[getdave]

My view on this is that we're auto-creating a Post in the users' database so we should allow them to opt out of that if they'd like. I don't have a super strong feeling about it but I thought it was worth a discussion.

[scruffian]

We only create the post if they use the block though, so it doesn't feel super intrusive to me.