fix(ui): fix for flutter 3.13

Signed-off-by: Sahil Kumar <xdsahil@gmail.com>

packages/stream_chat_flutter/lib/src/avatars/group_avatar.dart

@@ -66,7 +66,8 @@ class StreamGroupAvatar extends StatelessWidget {
     Widget avatar = GestureDetector(
       onTap: onTap,
       child: ClipRRect(
-        borderRadius: borderRadius ?? previewTheme?.borderRadius,
+        borderRadius:
+            borderRadius ?? previewTheme?.borderRadius ?? BorderRadius.zero,
         child: Container(
           constraints: constraints ?? previewTheme?.constraints,
           decoration: BoxDecoration(color: colorTheme.accentPrimary),

packages/stream_chat_flutter/lib/src/avatars/user_avatar.dart

@@ -86,7 +86,8 @@ class StreamUserAvatar extends StatelessWidget {
 
     final backupGradientAvatar = ClipRRect(
       borderRadius: borderRadius ??
-          streamChatTheme.ownMessageTheme.avatarTheme?.borderRadius,
+          streamChatTheme.ownMessageTheme.avatarTheme?.borderRadius ??
+          BorderRadius.zero,
       child: streamChatConfig.defaultUserImage(context, user),
     );
 

packages/stream_chat_flutter/lib/src/channel/stream_channel_avatar.dart

@@ -111,7 +111,8 @@ class StreamChannelAvatar extends StatelessWidget {
       initialData: channel.image,
       builder: (context, channelImage) {
         Widget child = ClipRRect(
-          borderRadius: borderRadius ?? previewTheme?.borderRadius,
+          borderRadius:
+              borderRadius ?? previewTheme?.borderRadius ?? BorderRadius.zero,
           child: Container(
             constraints: constraints ?? previewTheme?.constraints,
             decoration: BoxDecoration(color: colorTheme.accentPrimary),