[MathiasReichardt]

• treat enum containing null as nullable, e.g. {"enum": [..., null]} without type
• treat oneOf/anyOf with a null branch as nullable
• skip the non-null check for fields without type, enum or const, e.g. {}
• mark non-nullable fields as required, so the asterisk shows why Submit is disabled
• add form level tests that run each schema shape through the schema simplifier
• regression from Add file upload action parameters #32, which enforced non-null on every field and disabled Submit