Firebase Analytics: unable to clear user id in debug view

### Step 1: Describe your environment

* Xcode version: 11.2
* Firebase SDK version: 6.15.0
* Firebase Component: analytics
* Component version: 6.15.0
* Installation method: `CocoaPods`

### Step 2: Describe the problem

#### Steps to reproduce:

I am trying to clear the user id of the current user after a user logs out of my app. In my log out sequence I call the following:

Analytics.setUserID(nil)
In the documentation of this method it says:

> The user ID to ascribe to the user of this app on this device, which must be non-empty and no more than 256 characters long. **Setting userID to nil removes the user ID.**

So as far as I can tell I'm clearing it correctly.

However.

When I am in debugView on firebase console, I see no indication that the user id has been cleared and when I look at subsequent events after clearing the user id, I can see under user properties, the old user id of the logged out user. I would expect this field to be empty here.

Not sure if this is a bug on debug view of the firebase console or if I'm doing something wrong?

It should be noted that when I try to set the user id as such below, I can see the user id being set in debug view and events having that id under user properties:

Analytics.setUserID("1234")