fixed handling if user is not found

sharepoint/api/src/main/java/de/muehlencord/shared/sharepoint/api/usergroup/SPUser.java

@@ -40,6 +40,8 @@ public class SPUser extends SPJaxbObject<User> {
 
         if (returnValue == null) {
             return null;
+        } else if (returnValue.equals("")) {
+            return null;
         } else {
             return returnValue.substring(7);  // drop [i:0#.w|
         }
@@ -56,7 +58,7 @@ public class SPUser extends SPJaxbObject<User> {
     public String getDisplayName() {
         return getValue().displayName;
     }
-    
+
     public String getEmail() {
         return getValue().email;
     }