docs: clarify OSQP empty constraints

[Haihan-Jiang]

Addresses #570.

What

• Clarify that missing OSQP equality or inequality constraint families should be passed as None, not as empty constraint arrays.
• Document that at least one constraint family must be provided for OSQP; fully unconstrained QPs should use the unconstrained QP path instead.
• Mirror that guidance in the OSQP.run docstring so the generated API docs include it.

Testing

• git diff --check
• python3.12 -m compileall -q jaxopt/_src/osqp.py
• Temporary venv smoke test importing OSQP and checking the updated run docstring
• Temporary venv smoke test solving a small QP with params_eq=None and params_ineq=(G, h)

The full Sphinx docs build was not run locally because the docs requirements install the full documentation stack, including TensorFlow/Flax.