Fix null reference exception in Util script. (#76)

Co-authored-by: jeffgamedev <jbrooks@gala.games>

Author: jeffgamedev

Assets/Thirdweb/Core/Scripts/Utils.cs

@@ -110,7 +110,7 @@ public static bool IsWebGLBuild()
 
         public static string ReplaceIPFS(this string uri, string gateway = "https://gateway.ipfscdn.io/ipfs/")
         {
-            if (uri.StartsWith("ipfs://"))
+            if (!string.IsNullOrEmpty(uri) && uri.StartsWith("ipfs://"))
                 return uri.Replace("ipfs://", gateway);
             else
                 return uri;