Fix a few cases where ffmpeg produces no output (#3161)

* Treat no output from ffmpeg as an error condition
* Distinguish file vs. video duration, and use later where appropriate
* Check for empty file in generateFile

Co-authored-by: WithoutPants <53250216+WithoutPants@users.noreply.github.com>

pkg/scene/generate/generator.go

@@ -83,6 +83,16 @@ func (g Generator) generateFile(lockCtx *fsutil.LockContext, p Paths, pattern st
 		return err
 	}
 
+	// check if generated empty file
+	stat, err := os.Stat(tmpFn)
+	if err != nil {
+		return fmt.Errorf("error getting file stat: %w", err)
+	}
+
+	if stat.Size() == 0 {
+		return fmt.Errorf("ffmpeg command produced no output")
+	}
+
 	if err := fsutil.SafeMove(tmpFn, output); err != nil {
 		return fmt.Errorf("moving %s to %s", tmpFn, output)
 	}
@@ -142,5 +152,9 @@ func (g Generator) generateOutput(lockCtx *fsutil.LockContext, args []string) ([
 		return nil, fmt.Errorf("error running ffmpeg command <%s>: %w", strings.Join(args, " "), err)
 	}
 
+	if stdout.Len() == 0 {
+		return nil, fmt.Errorf("ffmpeg command produced no output: <%s>", strings.Join(args, " "))
+	}
+
 	return stdout.Bytes(), nil
 }