Check if a method/property is defined or not before calling

I am a newbie to RoR. I am trying to figure out how to check if a property is defined or not in the environment file(development.rb in this case).

We have a property defined in development.rb file, something like:

config.user = 'test-user'

Now in the code, I use it by calling:

 Rails.application.config.user

which gives me the required value.

But the problem is this configuration may be disabled sometimes. So, I want to check if this property is defined or not before assigning it. Something like

user_name = (if Rails.application.config.user is available)? 
             Rails.application.config.user : 'some_other_value'

I tried defined? and respond_to but did not work.

Any help/suggestions are appreciated. Thanks!