Fix using `try` on delegators in CI config entries

See:

https://github.com/rails/rails/commit/af53280a4b5b3323ac87dc60deb2b1b781197b2b

lib/gitlab/ci/config/node/configurable.rb

@@ -51,12 +51,12 @@ module Gitlab
             def helpers(*nodes)
               nodes.each do |symbol|
                 define_method("#{symbol}_defined?") do
-                  @entries[symbol].specified?
+                  @entries[symbol].specified? if @entries[symbol]
                 end
 
                 define_method("#{symbol}_value") do
                   raise Entry::InvalidError unless valid?
-                  @entries[symbol].try(:value)
+                  @entries[symbol].value if @entries[symbol]
                 end
 
                 alias_method symbol.to_sym, "#{symbol}_value".to_sym