BUG: fix `Result.to_arviz` if arviz is not installed.

Result.to_arviz raises a warning if arviz is not installed but then raises an error because az is not defined, see: https://git.ligo.org/lscsoft/bilby/-/blob/master/bilby/core/result.py?ref_type=heads#L1696

If the code catches the ImportError, I think it would make more sense for it not to error later on.