Do not check only for the Rails constant

This constant may be define for auxiliar gems like rails-html-sanitizer
and these methods call will fail.

activerecord/lib/active_record/migration.rb

@@ -39,7 +39,7 @@ def initialize(name)
 
   class PendingMigrationError < MigrationError#:nodoc:
     def initialize
-      if defined?(Rails)
+      if defined?(Rails.env)
         super("Migrations are pending. To resolve this issue, run:\n\n\tbin/rake db:migrate RAILS_ENV=#{::Rails.env}")
       else
         super("Migrations are pending. To resolve this issue, run:\n\n\tbin/rake db:migrate")

activesupport/lib/active_support/core_ext/kernel/debugger.rb

@@ -3,7 +3,7 @@ module Kernel
     # Starts a debugging session if the +debugger+ gem has been loaded (call rails server --debugger to load it).
     def debugger
       message = "\n***** Debugger requested, but was not available (ensure the debugger gem is listed in Gemfile/installed as gem): Start server with --debugger to enable *****\n"
-      defined?(Rails) ? Rails.logger.info(message) : $stderr.puts(message)
+      defined?(Rails.logger) ? Rails.logger.info(message) : $stderr.puts(message)
     end
     alias breakpoint debugger unless respond_to?(:breakpoint)
   end

activesupport/lib/active_support/deprecation/behaviors.rb

@@ -20,7 +20,7 @@ class Deprecation
 
       log: ->(message, callstack) {
         logger =
-            if defined?(Rails) && Rails.logger
+            if defined?(Rails.logger) && Rails.logger
               Rails.logger
             else
               require 'active_support/logger'