Fix occasional perma loading on explore page

[39bytes]

On the explore page, if you click on a course and press back + scroll down quickly, occasionally it will get permanently stuck in a loading state. (I can reproduce this quite easily on prod)

I think this happens because there's a race condition between two network requests, one for fetching page 1 and the other for fetching page 2, which somehow leads to courses being set to undefined after they resolve?
I tried to fix this by just falling back to the batch's courses instead of using the null chaining operator.
As far as I can tell from local testing with network throttling, it's fixed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.21%. Comparing base (811f72c) to head (5a0a98c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #627   +/-   ##
=======================================
  Coverage   86.21%   86.21%           
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  Files          45       45           
  Lines        6425     6425           
=======================================
  Hits         5539     5539           
  Misses        886      886           

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