jobs: ignore missing blob during virus scan

We currently have many failed VirusScannerJob enqueued, because the
underlying blob is missing.

This PR fixes the issue by discarding the job in those cases (because if
the blob is gone, the job is never going to succeed).

The implementation is based on a similar issue encoutered by the
ActiveStorage::AnalyzeJob. See 06f8baf73c

app/jobs/virus_scanner_job.rb

@@ -1,4 +1,6 @@
 class VirusScannerJob < ApplicationJob
+  discard_on ActiveRecord::RecordNotFound
+
   def perform(blob)
     metadata = extract_metadata_via_virus_scanner(blob)
     blob.update!(metadata: blob.metadata.merge(metadata))

spec/jobs/virus_scanner_job_spec.rb

@@ -35,4 +35,14 @@ RSpec.describe VirusScannerJob, type: :job do
 
     it { expect(champ.reload.piece_justificative_file.virus_scanner.infected?).to be_truthy }
   end
+
+  context "when the blob has been deleted" do
+    before do
+      Champ.find(champ.id).piece_justificative_file.purge
+    end
+
+    it "ignores the error" do
+      expect { subject }.not_to raise_error
+    end
+  end
 end