What should I put in the license field of package.json if my code is only for use by the company I work for?

Use UNLICENSED per the npm docs:

Finally, if you do not wish to grant others the right to use a private or unpublished package under any terms:

{
  "license": "UNLICENSED"
}

_{This is not to be confused with the license that was confusingly called “The Unlicense”.}