Wrong status code returned for sketch/file that does not exist

[thegitone23]

Nature of issue?

• Found a bug

Details about the bug:

• Web browser and version: Firefox
• Operating System: Linux
• Steps to reproduce this bug:
  Go to https://editor.p5js.org/sketches/any_id/assets/any_file and 502 is returned.
  The problem is caused by the fact that the function getFileContent which is supposed to handle this route isn't verifying whether the returned object from database is null or not and it's also not checking for undefined asset name.

Because the sketch/file does not exist, a 404 error should be returned.

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[thegitone23]

I am doing a pr to fix this issue
#1265