fix: video thumbnails on web null error

packages/stream_chat_flutter/lib/src/message_input/quoted_message_widget.dart

@@ -276,7 +276,7 @@ class _ParseAttachments extends StatelessWidget {
       'video': (_, attachment) {
         return StreamVideoThumbnailImage(
           key: ValueKey(attachment.assetUrl),
-          video: attachment.file?.path ?? attachment.assetUrl!,
+          video: attachment.file?.path ?? attachment.assetUrl,
           constraints: BoxConstraints.loose(const Size(32, 32)),
           errorBuilder: (_, __) => AttachmentError(
             constraints: BoxConstraints.loose(const Size(32, 32)),

packages/stream_chat_flutter/lib/src/message_input/stream_message_input.dart

@@ -1194,7 +1194,7 @@ class StreamMessageInputState extends State<StreamMessageInput>
                   104,
                 ),
               ),
-              video: (attachment.file?.path ?? attachment.assetUrl)!,
+              video: attachment.file?.path ?? attachment.assetUrl,
             ),
             Positioned(
               left: 8,