Fix NoMethodError in TypeProf::Core::Type::Singleton

[sinsoku]

I encountered the following error:

/Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:10:in `block in typecheck_for_module': undefined method `args' for an instance of TypeProf::Core::Type::Singleton (NoMethodError)

            f_args.zip(ty.args) do |f_arg_node, a_arg_ty|
                         ^^^^^
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/graph/vertex.rb:11:in `each_key'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/graph/vertex.rb:11:in `each_type'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:5:in `typecheck_for_module'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:669:in `typecheck'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:483:in `block in typecheck'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:482:in `each'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:482:in `typecheck'

The args method is only defined on Type::Instance, so it adds a conditional expression to avoid the error.