No support for constraint free problems

The following snippet raises an error, but I believe it is a properly formed QP, and there is a solution x = [0, 0, 0, 0].

```python
qp = mpax.create_qp(jnp.eye(4), jnp.zeros(0), jnp.zeros((0, 4)), jnp.zeros(0), jnp.zeros((0, 4)), jnp.zeros(0), -jnp.ones(4), jnp.ones(4))
solver = mpax.r2HPDHG()
solver.optimize(qp)
```