Hide UI when MapInteractionMode message is empty.

[na9da]

What this PR does

Previously when MapInteractionMode message is empty, Terria shows a blank header with padding like below.

This PR fixes is it so that nothing is shown when message is empty. This useful when rendering some other UI using customUi, example to show an ActionBar instead of the regular MapInteractionMode UI.

Before:

Test me

Hard to provide a CI link as it requires a custom mode for testing.
Existing tools like Measurement tools that provide a message should still continue to work.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.