sync: Don't overwrite theme image if it exists

admin/sync/exercices.go

@@ -400,10 +400,15 @@ func SyncExercice(i Importer, theme *fic.Theme, epath string, dmap *map[int64]*f
 					e.BackgroundColor, _ = getBackgroundColor(filePath)
 
 					// If the theme has no image yet, use the first exercice's image found
+					if theme != nil && theme.Image == "" {
 						theme.Image = e.Image
 						_, err := theme.Update()
-
+						if err != nil {
 							return nil, err
+						}
+					}
+
+					return nil, nil
 				}); err != nil {
 				errs = multierr.Append(errs, NewExerciceError(e, fmt.Errorf("unable to import heading image: %w", err)))
 			}