Cask::DSL::DependsOn: add empty?, present? methods

`#present?` is called on a `DependsOn` object in `Cask::DSL` and this
is seemingly deferred to the underlying hash object but Sorbet doesn't
understand this kind of `SimpleDelegator` magic. This adds `empty?`
and `present?` methods that explicitly interact with the hash in a
way that Sorbet can understand.

Library/Homebrew/cask/dsl/depends_on.rb

@@ -82,6 +82,12 @@ module Cask
 
         @arch.concat(arches.map { |arch| VALID_ARCHES[arch] })
       end
+
+      sig { returns(T::Boolean) }
+      def empty? = T.let(__getobj__, T::Hash[Symbol, T.untyped]).empty?
+
+      sig { returns(T::Boolean) }
+      def present? = !empty?
     end
   end
 end