fix(cascader): 修复级联组件无法选中值为 0 的选项 (#2144)

修复在级联组件单选状态下，选中项value为0时，label不能正常显示

fix #2143

src/cascader/core/helper.ts

@@ -18,7 +18,7 @@ export function getSingleContent(cascaderContext: CascaderContextType): string {
   const {
     value, multiple, treeStore, showAllLevels,
   } = cascaderContext;
-  if (multiple || !value) return '';
+  if (multiple || (typeof value !== 'number' && !value)) return '';
 
   if (Array.isArray(value)) return '';
   const node = treeStore && treeStore.getNodes(value as TreeNodeValue | TreeNode);