fix: [Image] Fix the problem that the custom Header cannot be displayed when previewing a single image (#1298)

packages/semi-ui/image/preview.tsx

@@ -168,7 +168,7 @@ export default class Preview extends BaseComponent<PreviewProps, PreviewState> {
         return (
             <PreviewContext.Provider
                 value={{
-                    isGroup: finalSrcList.length > 1,
+                    isGroup: true,
                     previewSrc: finalSrcList,
                     titles: titles,
                     currentIndex,