Add missing test per JaCoCo test coverage

Author: garydgregory

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

@@ -168,6 +168,7 @@ void testOfNonNull() {
 
     @Test
     void testPairOfMapEntry() {
+        assertSame(ImmutablePair.nullPair(), ImmutablePair.of(null));
         final HashMap<Integer, String> map = new HashMap<>();
         map.put(0, "foo");
         final Entry<Integer, String> entry = map.entrySet().iterator().next();