ContentNodeSlim "popular" view can return an empty string as response

Observed behavior

Related to #5407, a request to /api/content/contentnode_slim/popular/?by_role=true&include_fields=num_coach_contents&contentCacheKey=1556915747

Is returning a 200 response with "" on my local version, which in turn causes the problem in #5407. The other detail view (“resume” and “next_steps”) are working correctly.

In the Network inspector in chrome dev tools

I do not know what, specifically, is causing the “popular” view to return nothing.

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Expected behavior

Assuming, this is a legitimate response to, for example, not having any “popular” resources, a call to this endpoint returns something acceptable to the Api Resource class, like [].

User-facing consequences

Servers with the same conditions as mine (whatever they are) will not have a working “Learn > Recommended” page.

Errors and logs

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Steps to reproduce

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Context

0.12. develop

[rtibbles]

Right, either would work.

[jonboiser]

Implemented the solution above in #5486. In local development mode, the use-the-cache branch should always be bypassed, and the view returns Response(serializer.data).