fix: errors during swagger api-docs generation

[pomegranited]

This PR fixes two errors seen during the automatic Swagger API docs generation.

Part of: #272
Private-ref: FAL-4039

Testing instructions

1. Install this branch in your edx-platform devstack
2. Visit the Studio API docs page, e.g http://local.openedx.io:8001/api-docs/
   Note that the page renders now without error.

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[navinkarkera]

@pomegranited Looks good, but I had to make following changes in edx-platform: openedx/openedx-platform#36189 to get it working.