AssertTests: add test case

AssertTests.isNullWithNonNullObject

Signed-off-by: Vincent Stradiot <vincentstradiot@hotmail.com>

spring-core/src/test/java/org/springframework/util/AssertTests.java

@@ -109,6 +109,12 @@ class AssertTests {
 		Assert.isNull(null, "Bla");
 	}
 
+	@Test
+	void isNullWithNonNullObject() {
+		assertThatIllegalArgumentException().isThrownBy(() -> Assert.isNull("foo", "Bla"))
+				.withMessage("Bla");
+	}
+
 	@Test
 	void isNullWithMessageSupplier() {
 		Assert.isNull(null, () -> "enigma");