The reason I ask is that I tried to reproduce your problem and failed: I got null instead of empty string.
I suspect the problem is in the way you save your table entries. Maybe, your column is created dynamically, and you’ve saved an empty string into it in some way.
Anyway, we need some more information about how you run into this problem in order to reproduce it.
I did some further examination and realized I had a legacy file that contained another version of the MemberMain function, which was overriding the one I sent you. It contained a different initialization of joinDate, i.e. :
this.joinDate = args.joinDate || "";
Thanks for your help, and sorry to waste your time.