fix(a32nx/fms): disallow create waypoint ident edits, not in db handling

[tracernz]

Fixes #[issue_no]

Summary of Changes

• Fixed an issue that prevented the create waypoint page being shown for NOT IN DATABASE.
• Fixed an issue that prevented the RETURN working on the create waypoint page.
• Removed null from the PilotWaypoint union type. | null should be explicit when it is needed, and it is not wanted except in the A380 code.
• Add type and fix a bug it uncovered with the create waypoint callback.
• The change I intended: Disallow editing of the ident once entered on the create waypoint page.

Screenshots (if necessary)

References

Additional context

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Testing instructions

• Go to the stored waypoints page and enter the ident for a new waypoint; try to press the LSK next to the ident a 2nd time and ensure you get NOT ALLOWED.
• Create a flightplan and insert a waypoint on the F-PLN page that doesn't exist. Ensure you get NOT IN DATABASE in the scratchpad, and the create waypoint page. Press the RETURN LSK and ensure you go back to the F-PLN page with nothing new inserted.
• Repeated the above but enter a location for the new waypoint and press STORE. Ensure that the waypoint is inserted into the plan and you are returned to the F-PLN page.

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