[improvement] props default value (#2183)

packages/image-preview/ImagePreview.vue

@@ -59,19 +59,13 @@ export default create({
   },
 
   props: {
+    images: Array,
+    startPosition: Number,
     showIndicators: Boolean,
-    images: {
-      type: Array,
-      default: () => []
-    },
     loop: {
       type: Boolean,
       default: true
     },
-    startPosition: {
-      type: Number,
-      default: 0
-    },
     overlay: {
       type: Boolean,
       default: true