sb1.append("TEST").append((CharSequence)null) , this works as expected. 
But if the second append is called after toString(), "null" is then not appended.

public class TestAppendNull {
    
    public static void main(String[] args) {
        
        StringBuffer sb1 = new StringBuffer();
        sb1.append("TEST");
        System.out.println(sb1.toString());

        sb1.append((CharSequence)null);
        System.out.println("Expected: TESTnull");
        System.out.println("Result: " + sb1.toString());
    }

}

bash-3.2$ $JAVA_HOME/bin/java TestAppendNull
TEST
Expected: TESTnull
Result: TEST


SQE test 
java_lang/Bug4812591
failed due to this issue