builder/openstack: Fix interface conversion panic when attempting to find a volume when not using volumes

builder/openstack/step_create_image.go

@@ -92,16 +92,18 @@ func (s *stepCreateImage) Run(ctx context.Context, state multistep.StateBag) mul
 		return multistep.ActionHalt
 	}
 
-	volume := state.Get("volume_id").(string)
-	if len(config.ImageMetadata) > 0 && s.UseBlockStorageVolume {
-		err = volumeactions.SetImageMetadata(blockStorageClient, volume, volumeactions.ImageMetadataOpts{
-			Metadata: config.ImageMetadata,
-		}).ExtractErr()
-		if err != nil {
-			err := fmt.Errorf("Error setting image metadata: %s", err)
-			state.Put("error", err)
-			ui.Error(err.Error())
-			return multistep.ActionHalt
+	if s.UseBlockStorageVolume {
+		volume := state.Get("volume_id").(string)
+		if len(config.ImageMetadata) > 0 {
+			err = volumeactions.SetImageMetadata(blockStorageClient, volume, volumeactions.ImageMetadataOpts{
+				Metadata: config.ImageMetadata,
+			}).ExtractErr()
+			if err != nil {
+				err := fmt.Errorf("Error setting image metadata: %s", err)
+				state.Put("error", err)
+				ui.Error(err.Error())
+				return multistep.ActionHalt
+			}
 		}
 	}