In our existing diagnostics, our message is just the diagnostic
ID, and the message goes to the annotation. In reality, the
diagnostic can have its own message distinct from the optional
messages associated with an annotation.
In order to make the outputs match, we do a small tweak here:
when the main diagnostic message is empty, we drop the colon
after the diagnostic ID.
I expect that we'll want to rejigger this output format more
in the future, but for now this was a very simple change to
preserve the status quo.
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