Handle nil expiration_time_millis

`data.fetch("expiration_time_millis", 0)` will default to 0 only if the key is not present. If the key is present but the associated value is nil, then prior to this change the code would raise as follows:

```
NoMethodError (undefined method `/' for nil:NilClass

          expires_at:    data.fetch("expiration_time_millis", 0) / 1000
                                                                 ^):

```

This commit updates the code to default to 0 if the key is not found or if the value is nil.

With the existing token stores, this situation may not arise, but it's a particular issue when using a custom token store where values could be nil, like a database.

Author: sampart

lib/googleauth/user_authorizer.rb

@@ -156,7 +156,7 @@ def get_credentials user_id, scope = nil
           scope:         data["scope"] || @scope,
           access_token:  data["access_token"],
           refresh_token: data["refresh_token"],
-          expires_at:    data.fetch("expiration_time_millis", 0) / 1000
+          expires_at:    (data.fetch("expiration_time_millis") || 0) / 1000
         )
         scope ||= @scope
         return monitor_credentials user_id, credentials if credentials.includes_scope? scope