Remove colons after empty strings in diff labels

[colin-grant-work]

Signed-off-by: Colin Grant colin.grant@ericsson.com

What it does

In the screenshots for #9765, it is clear that diff widgets can wind up with rather odd initial : characters. This is due to faulty logic in the label provider for DiffUris, which doesn't check whether a .displayName is available before creating certain prefixes that use it.

How to test

1. Open two or more editors.
2. Use the Open Editors widget context menu to select one for comparison and compare another with that selection.
3. Observe that the title of the diff widget that appears does not contain a leading : character unless that colon is preceded by actual text.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

I confirmed that the diff label bug of #9765 is resolved with the issue 👍

[msujew]

I can confirm that the issue exists on master is addressed nicely by this change.