Make name field of _UserInfo nullable. Fix #3424

[talisk]

Fix #3424 .

The PR #3427 only fix the dart error. But actually pub cannot retrieve the user info.

As @bsutton said.

The symptom is that the 'name' field is not being returned by the google auth mechanism.
I've tried pub logout and then pub login with the same results.
Is there some chance that google no longer return the 'name' field in the user info response?

Reading the google documentation:
https://developers.google.com/identity/protocols/oauth2/openid-connect#server-flow
name - The user's full name, in a displayable form. Might be provided when: The request scope included the string "profile"
The ID token is returned from a token refresh. When name claims are present, you can use them to update your app's user records.

Note that this claim is never guaranteed to be present.
Note the last sentence.

We should make the name filed nullable.

[jonasfj]