[ruby/rubygems] Return exit status 1 only when the config value is nil

When using something like `bundle config set foo false`, the config value is converted via `Bundler::Settings#converted_value`, so the stored value becomes `false` instead of the string `"false"`. Because of that, passing `Bundler.settings[name]` directly to an `if` statement can execute `exit 1` even when the value is actually configured.

Since config values do not appear to become `nil` explicitly, use `nil?` to determine whether the value is configured.

ruby/rubygems@8fd32cb611

Author: willnet

lib/bundler/cli/config.rb

@@ -97,10 +97,10 @@ def run
             confirm(name)
           end
 
-          if current_value
-            return
-          else
+          if current_value.nil?
             exit 1
+          else
+            return
           end
         end