Fix: Check for None before casting with complex PETCs

[nikolasborrel]

Changes

Check if x0 is None before casting to complex values in function jax_solve(A, b, x0, precond). This can happen when inverse problems are considered and jax_solve(...) is called from

adjoint_vec = linear_solver(A.transpose(), v_vec, None, adjoint_solver_options)

inside def implicit_vjp(...) in solver.py

This is a fix for changes made in #72 implementing support for PETSc with complex number.