Add serializer unit tests

[AetherUnbound]

Problem

We do not currently have unit tests for any of the API serializers (api/catalog/api/serializers/*).

Description

We should add thorough unit tests to these serializers, focusing on the base functions in the media_serializers.py file.

Additional context

This came up as part of WordPress/openverse-api#526.

Implementation

• [ ] 🙋 I would be interested in implementing this feature.