Navigation: Fix warning when stretch justification is used

[andrewserong]

What?

Fixes the warning issue reported in #50550

In the Navigation block's PHP, check that the provided justification value is present in the mapping array before attempting to access it.

Why?

Since stretch was introduced as a valid value, when stretch is set on the Navigation block, it throws a warning when rendered on the site frontend.

This PR doesn't implement proper support for stretch in this block — it's just looking at resolving this PHP warning.

How?

• Check the value exists before attempting to access it

Testing Instructions

1. Add a Navigation block
2. Set it to vertical orientation
3. Select stretch content justification (the right-most option)
4. Save the block
5. Load the post / page / site on the site's frontend
6. In PHP logs, note that with this PR applied there should be no warning logged.
7. stretch doesn't really do anything for this block yet (I don't think), but double-check that left, centre, right justifications still work as expected

Testing Instructions for Keyboard

Screenshots or screencast

This is the warning that should not be logged with this PR applied:

[aristath]

Does what it says on the tin.
LGTM 👍

[andrewserong]

Thanks for the review!

[tellthemachines]

Thanks for fixing this! I've been meaning to update the layout logic in the Nav block since #44600, as we shouldn't need the custom properties any longer, but there's always so much to do 😅

[andrewserong]

but there's always so much to do 😅

Oh, I hear ya! My to-do list grows faster than my to-done list 😅