Had to fix default_value usage

src/cligen/command/define_command_initializer.cr

@@ -14,7 +14,7 @@ module CliGen
                 raise CliGen::FlagNotFoundError.new("{{@type.name}}\#{{@def.name}} : No flag found for \"{{var.name}}\"")
               end
             {% else %}
-              {% raise "ERROR : #{@type.name}#{@def.name} : Instance Variable(#{var.name}) is not handled by CliGen and does not have a default value" unless var.default_value %}
+              {% raise "ERROR : #{@type.name}#{@def.name} : Instance Variable(#{var.name}) is not handled by CliGen and does not have a default value" if var.default_value.nil? %}
               @{{var.id}} = {{var.default_value}}
             {% end %}
           {% end %}