--corrected the isUnchecked method to check for null (#1079)

--created test for null case

src/main/java/org/apache/commons/lang3/exception/ExceptionUtils.java

@@ -641,7 +641,7 @@ public class ExceptionUtils {
      * @since 3.13.0
      */
     public static boolean isUnchecked(final Throwable throwable) {
-        return !isChecked(throwable);
+        return throwable != null && (throwable instanceof Error || throwable instanceof RuntimeException);
     }
 
     /**

src/test/java/org/apache/commons/lang3/exception/ExceptionUtilsTest.java

@@ -635,6 +635,11 @@ public class ExceptionUtilsTest extends AbstractLangTest {
         assertTrue(ExceptionUtils.isChecked(new TestThrowable()));
     }
 
+    @Test
+    public void testIsUnchecked_null() {
+        assertFalse(ExceptionUtils.isUnchecked(null));
+    }
+
     @Test
     public void testIsUnchecked_checked() {
         assertFalse(ExceptionUtils.isUnchecked(new IOException()));