fix: Set instance variables at initialization to avoid spamming rake warnings

lib/googleauth/credentials.rb

@@ -257,6 +257,9 @@ module Google
         CredentialsLoader.warn_if_cloud_sdk_credentials @client.client_id
         @project_id ||= CredentialsLoader.load_gcloud_project_id
         @client.fetch_access_token!
+        @env_vars = nil
+        @paths = nil
+        @scope = nil
       end
       # rubocop:enable Metrics/AbcSize
 

lib/googleauth/signet.rb

@@ -66,7 +66,7 @@ module Signet
       end
 
       def on_refresh &block
-        @refresh_listeners ||= []
+        @refresh_listeners = [] unless defined? @refresh_listeners
         @refresh_listeners << block
       end
 
@@ -84,7 +84,7 @@ module Signet
       end
 
       def notify_refresh_listeners
-        listeners = @refresh_listeners || []
+        listeners = defined?(@refresh_listeners) ? @refresh_listeners : []
         listeners.each do |block|
           block.call self
         end