Fix members length check in JsDuck::Lint.

    # print warnings for enums with no values

    def warn_empty_enums

      @relations.each do |cls|

        if cls[:enum] && cls[:members][:property].length == 0

        if cls[:enum] && cls[:members].length == 0

          warn(:enum, "Enum #{cls[:name]} defined without values in it", cls)

        end

      end