Update test

exercises/isbn-verifier/src/test/java/IsbnVerifierTest.java

         assertFalse(isbnVerifier.isValid("3-598-21515-X"));
     }
 
+    @Ignore("Remove to run test")
+    @Test
+    public void emptyIsbn() {
+        assertFalse(isbnVerifier.isValid(""));
+    }
+
 }