[Task] Handle "no target" selection on Dashboard

I say "at least for now" because we should probably actually handle the `NO_TARGET` selection case nicely and allow the dashboard to be created and a skeleton or empty state version of it rendered when the user hasn't selected any target. But even after we have that I think that starting a recording should still be an explicit opt-in action.

_Originally posted by @andrewazores in https://github.com/cryostatio/cryostat-web/issues/862#issuecomment-1432301516_

Search the dashboard implementation for the NO_TARGET selection path and determine how dashboard creation and rendering currently depend on a selected target. Trace the recording-start action as well. Done means a dashboard can render a skeleton or empty state without a target, while starting a recording remains an explicit opt-in action.

Written by the indexing model from the issue text.