Fatal warnings prints None when detail not provided

Summary

When a fatal-warning is triggered without detail, the exit message prints None.

Steps to reproduce

Configure a warning as fatal.

Raise it somewhere in a source with self.warn("brief information", warning_token=CoreWarnings.<warning>)

What is the current bug behavior?

Buildstream will exit and print

Brief information
None

What is the expected correct behaviour?

Brief information

• BuildStream version affected: master