Add missing test per JaCoCo test coverage

Author: garydgregory

src/test/java/org/apache/commons/lang3/tuple/MutablePairTest.java

@@ -116,6 +116,8 @@ void testOfNonNull() {
 
     @Test
     void testPairOfMapEntry() {
+        assertNull( MutablePair.of(null).getLeft());
+        assertNull( MutablePair.of(null).getRight());
         final HashMap<Integer, String> map = new HashMap<>();
         map.put(0, "foo");
         final Entry<Integer, String> entry = map.entrySet().iterator().next();