Fixes #177. 🦕

This PR fixes the error in RowIterator.to_arrow() when the method should create and use a BQ Storage client, but the optional google-cloud-bigquery-storage dependency is not installed and thus the client cannot be created.

PR checklist:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Thanks for getting this together.

Independent of this change, should we consider something like a nox session that's parameterized to run a subset of the system or snippet tests with different combinations of optional deps?

Depending on the number of dependency combinations that we want to test and the slowness of the target tests, it might be an overkill to run them for every PR, but how about setting up regular nightly runs? And possibly also run such an extensive suite before every release?