Handle when an alert message contains links to files outside of the repository source

[robertbrignull]

Closes #2622

Fixes support for MRVA using queries where a placeholder link goes to outside of the source archive. In this case we have a location without a region and this was causing a crash because we were naughtily assuming fields to be defined.

Rendering of alerts now works and the message placeholder is rendered as a regular string instead of a link:

Use the following query to reproduce the bug and observe that it now works:

/**
 * @kind problem
 */

import java

// Pick arbitrary element which is part of source; chose EnumType here because projects
// likely don't have that many enum types
// TypeString comes from the JDK and is not part of the source of the project
from EnumType t, TypeString notPartOfSource
where t.fromSource()
select t, "message $@", notPartOfSource, notPartOfSource.getName()

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[charisk]

Thanks for fixing this! I've not tested locally myself but the code changes make sense.

[robertbrignull]

I've not tested locally myself but the code changes make sense.

It was last week, but I did test locally, including with a message that does contain working links. I think it's all fine.