data.json files for courses without semester or year contain "" values instead of null values

[mbertrand]

Expected Behavior

Courses without a year or semester have a data.json file containing this:

{
  "year": null,
  "semester": null
}

Current Behavior

{
  "year": "",
  "semester": ""
}

Steps to Reproduce

https://draft-qa.ocw.mit.edu/courses/res-hs-002-chemistry-behind-the-magic-chemical-demonstrations-for-the-classroom/data.json