Prepare for Pydantic v2 migration

[r4victor]

Part of #1844

Pydantic v1 -compatible changes that prepare for Pydantic v2 migration:

• Set None default for all Optional fields: Missing None default on v2 makes the field required. On v1 there is no difference.
• Parse data from db/server via response everywhere – this is largely needed to cover all places with pydantic v1-v2 compatibility tests consistently.
• Use discriminator= for unions everywhere where possible as pydantic v2 now uses smart mode by default for parsing unions vs left-to-right parsing.