Fix display of image messages that lack thumbnails

Fixes https://github.com/vector-im/element-web/issues/18175

src/utils/MediaEventHelper.ts

@@ -67,6 +67,7 @@ export class MediaEventHelper implements IDestroyable {
     private prepareThumbnailUrl = async () => {
         if (this.media.isEncrypted) {
             const blob = await this.thumbnailBlob.value;
+            if (blob === null) return null;
             return URL.createObjectURL(blob);
         } else {
             return this.media.thumbnailHttp;