I use a collection of documents that I use to identify user groups. My intention is to only fill in the document id field with the user id without having other useless fields in
A solution that worked for me, as simple as it seems is setting the field you want to nullify to an empty string for strings and null for integers. No fancy stuff, if you want you can test it out by manually setting the values to null in your firebase firestore document browser.
this.itemDoc.update({item: ''});
or
this.itemDoc.update({item: null});
This was done using angularfire2, here's the link to the documentation : https://github.com/angular/angularfire2/blob/master/docs/firestore/documents.md