Make Noneable types make fields become optional by default
Before this change, fields were automatically made default only
if they were Shapes or Selectors. However, any Noneable type should be
optional and default to None. This came up during asset store/input
output manager exploration.
This is not a breaking change w/r/t to config schema, because this is
strictly more permissive. However this could potentially break anyone
who has built tooling on top of the config system.
Test Plan: BK
Reviewers: sandyryza, max, dgibson
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D5320