Support for adjoint simulations with no monitors (zero gradients)

[yaugenst-flex]

Idea is to allow functions like min() and max() in an objective function where some simulations (for example in an adjoint batch) might not get adjoint sources.

Approach:

• Generally allow make_adjoint_sim to return None instead of erroring.
• Handle whether or not we error if there are no adjoint sources in run and run_async, where by default we error in run and only warn in run_async:
  • In run, the user is probably running only a single simulation. If that gets no adjoint sources, then there is a problem with the setup. However, it is possible that a user might be running multiple simulations serially, in which case we do offer the option to allow "empty" adjoint sims.
  • In run_async, the user is expected to run a batch of multiple simulations, so it should generally be fine if some of those do not receive adjoint sources. We should probably error though if all simulations in a batch have no adjoint sources.