[memoization improvements 2/n] allow dagster execution with no steps.

Summary: Currently, we fail fast (and in different ways) when attempting to execute with no step keys. This unifies the meaning of empty step keys across the system: step_keys_to_execute=None means execute all steps, step_keys_to_execute=[] means empty steps.

Test Plan: Fix all tests to make sure that this is the case.

Reviewers: sandyryza, alangenfeld

Reviewed By: alangenfeld

Differential Revision: https://dagster.phacility.com/D8923