Fix sharepoint null reference

Sharepoint repo will throw exception if value is null

Author: robinhultman

Src/LookupUtility/Repository/SharepointLookupRepository.cs

@@ -37,7 +37,7 @@ public Dictionary<string, string> LoadList(string list)
                 item.FieldValues.TryGetValue("Key", out object keyName);
                 item.FieldValues.TryGetValue("Value", out object valueName);
 
-                dictionary.Add(keyName.ToString(), valueName.ToString());
+                dictionary.Add(keyName.ToString(), valueName as string);
             }
 
             return dictionary;