Why does stripe.Account.retrieve("") return an account, and what account is it?

I'm attempting to make a utility function to check if a user has submitted their onboarding details.

Ex:

@property
def is_onboarded(self):
    stripe = stripe_config()
    a = stripe.Account.retrieve(self.stripe_user_id)
    return a.details_submitted

If self.stripe_user_id is an empty string, it returns some account for an ID I can't track down (don't see it in the online Connect interface).

Ex:

>>> stripe.Account.retrieve('')
<Account account id=acct_abc123abc1235N at 0x264b6b6cd00> JSON: {
  "capabilities": {
    "acss_debit_payments": "active",
    ...

My simple question is, why? And, what account is it returning?