[FIX] Corrected Create method so that String UDI is correctly identified

- [FIX] Changed check of UDI type to check for GUID type and then String type as previously was checking for
GUID type twice. This resulted in the method never creating a String UDI if needed.
 - [ISSUE] Github Issue #7507 https://github.com/umbraco/Umbraco-CMS/issues/7507

src/Umbraco.Core/Udi.cs

@@ -316,7 +316,7 @@ namespace Umbraco.Core
 
             if (udiType == UdiType.GuidUdi)
                 return new GuidUdi(uri);
-            if (udiType == UdiType.GuidUdi)
+            if (udiType == UdiType.StringUdi)
                 return new StringUdi(uri);
 
             throw new ArgumentException(string.Format("Uri \"{0}\" is not a valid udi.", uri));