System Menu blocks do not have attributes

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For the specific test coverage still needed.

Also I see that the tests were updated is that because this attribute is now required? Won't this break existing sites that don't have it?

Opened an MR with a slightly different approach.

The existing patch will add an ID to every since block that is added via LB, which feels a bit overkill to solve this. Instead I've added a default ID that will be applied to system_menu_blocks if one does not exist. If people like this approach I'll add some tests

The Needs Review Queue Bot → tested this issue.

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Seems a few other block--system-menu-block.html.twig in core. Is there a place we can add an assertion to?