Simplify comparison

From https://doc.qt.io/qt-5/qdatetime.html#operator-eq-eq:
>Since 5.14, all invalid datetimes are equal to one another and differ from all other datetimes.

src/gui/transferlistsortmodel.cpp

@@ -51,10 +51,8 @@ namespace
         const bool isLeftValid = left.isValid();
         const bool isRightValid = right.isValid();
 
-        if (isLeftValid && isRightValid)
+        if (isLeftValid == isRightValid)
             return threeWayCompare(left, right);
-        if (!isLeftValid && !isRightValid)
-            return 0;
         return isLeftValid ? -1 : 1;
     }