mypy run storage

Summary:

• mypy methods in
  • in_memory
  • sql_run_storage: -> use deserialize_as instead of deserialize_json_to_dagster_namedtuple
• update methods which didn't pass type check, i.e. get_run_group, get_run_groups
  • guard empty pipeline run - error like https://gist.github.com/alangenfeld/8e394ccf12dda9abafd79c6d99d3d1f7 shouldn't happen
  • flatten nested defaultdict into two separate dicts for readability and easier mypy typing
• get_backfill should return Optional[PartitionBackfill] not PartitionBackfill

Test Plan: bk

Reviewers: max, alangenfeld, dgibson

Reviewed By: dgibson

Differential Revision: https://dagster.phacility.com/D9071