Issue Summary

Missing external IDs from schema and mapping.

In the documentation we see:
"If there isn't an external id provided in the schema and mapping, there will be one automatically generated for you. To find out what the external id is for an object type or object type attribute, you can call the GET /

/schema-and-mapping endpoint."

However, if an external ID is missing we are not able to use the returned data to directly patch the mapping for the external import as it will return Bad request due to missing external IDs.
It's important that we are able to use the returned schema and mapping data to patch mapping when necessary.

Steps to Reproduce

1. GET /{importSourceId}/schema-and-mapping endpoint.
2. With a result that is missing external IDs attempt to PATCH the mapping

Expected Results

Success

Actual Results

400 Bad Request
Mentions of missing external IDs

Workaround

Add external ID values when these are missing