[OpenAPI] Mark all not required request fields as nullable

lib/pleroma/web/api_spec/operations/account_operation.ex

lib/pleroma/web/api_spec/operations/app_operation.ex

lib/pleroma/web/api_spec/operations/filter_operation.ex

lib/pleroma/web/api_spec/operations/marker_operation.ex

...roma/web/api_spec/operations/pleroma_account_operation.ex

lib/pleroma/web/api_spec/operations/report_operation.ex

lib/pleroma/web/api_spec/operations/status_operation.ex

...pleroma/web/api_spec/operations/subscription_operation.ex