Add handling for parent channel being null

- Not certain if this is strictly required, but probably a good idea to have in case it is

Author: hamarb123

src/Modix.Bot/Responders/StarboardReactionResponder.cs

@@ -43,7 +43,7 @@ private async Task HandleReactionAsync(Cacheable<IUserMessage, ulong> cachedMess
             var isIgnoredFromStarboard = await designatedChannelService
                 .ChannelHasDesignation(channel.Guild.Id, channel.Id, DesignatedChannelType.IgnoredFromStarboard, default);
 
-            var isParentIgnoredFromStarboard = channel is SocketThreadChannel threadChannel && await designatedChannelService
+            var isParentIgnoredFromStarboard = channel is SocketThreadChannel { ParentChannel: not null } threadChannel && await designatedChannelService
                 .ChannelHasDesignation(channel.Guild.Id, threadChannel.ParentChannel.Id, DesignatedChannelType.IgnoredFromStarboard, default);
 
             var starboardExists = await designatedChannelService