Attempt user fetch only if patron access token is not null

[beberry]

Problem

The getPatreonUser function attempts to fetch user data as long
as access_token != ''. This check does not account for the access
token being null. Access token data can be set to null if a token
refresh failed. Because of this, the app might be attempting to refresh
user tokens even of no token is available.

Solution

Use empty which checks for both - null and ''.