Do not attempt to resolve an adapter_class if no adapter is present

Author: kmcphillips

activerecord/lib/active_record/database_configurations/database_config.rb

@@ -10,7 +10,7 @@ class DatabaseConfig # :nodoc:
 
       def self.new(...)
         instance = super
-        instance.adapter_class # Ensure resolution happens early
+        instance.adapter_class if instance.adapter # Ensure resolution happens early
         instance
       end