Navigation: Show the white appender/plus button when empty

[jasmussen]

When you insert a Navigation block and click "Start empty", it collapses to zero height because the container is empty (see also #38439).

Additionally, if you unselect the block in this state, it becomes invisible so you might forget there's a navigation there at all.

We should do for an empty navigation block the same as the Row block does, show the white plus button in this state:

That way, there would both be something to indicate an empty block, but it would also avoid collapsing.

[jasmussen]

As of recent changes, the empty navigation block doesn't collapse as shown in the GIF above, and while it doesn't show the big white appender as suggested, in effect this solves the issue!

Therefore, closing this one 🎉