SequenceLayout::toString throws ArithmeticException

If a sequence layout contains an element type with `byteSize()` zero, the SequenceLayout::toString method will throw an ArithmeticException (division by zero).

Reproduce:

    @Test
    public void testSequenceLayoutWithZeroLength() {
        SequenceLayout layout = MemoryLayout.sequenceLayout(0, JAVA_INT);
        assertEquals(layout.toString(), "[0:i4]");

        SequenceLayout nested = MemoryLayout.sequenceLayout(0, layout);
        assertEquals(nested.toString(), "[0:[0:i4]]"); // <- Fails!
    }