dm discover file-report prints an empty list when no report exists

[ClassicMMT]

What happens

dm discover file-report <run-id> prints [] on stdout, prints nothing on
stderr, and exits 0 in three conditions:

• the run is still in progress
• the run failed
• the run is a database discovery run, not a file discovery run

Why this is a problem

A completed file run that found no sensitive data gives the same output.
The caller cannot see the difference. An agent that calls file-report after
run start reads "the run scanned nothing" as "the run found nothing".

Cause

The server answers with HTTP 200 and an empty list in all three conditions.
The 404 handler in the command does not run.

Suggested fix

When the report is empty, get the run status.

• For a run that is not in a final state, abort with the hint from
  _abort_if_run_output_missing: "Discovery output is written once the run
  reaches a final state."
• For a database run, abort with a different message.

Source

Issue found here.