Fix Perl cannot string bind undef

Created by: AnaTofuZ

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@wing328 @yue9944882

The following Perl code will raise an exception

my %undef_test_order_data = (
    id     => undef,
    status => undef,
);
my $undef_test_order = WWW::OpenAPIClient::Object::Order->new->from_hash(\%undef_test_order_data);
lives_ok { JSON->new->convert_blessed->encode($undef_test_order) };

 Use of uninitialized value $data in concatenation (.) or string at lib/WWW/OpenAPIClient/Object/Order.pm line 200.
Use of uninitialized value $data in addition (+) at lib/WWW/OpenAPIClient/Object/Order.pm line 198.

This is because Perl's undef cannot do string concatenation or addition, but it does these when converting types to JSON. The way to deal with this is to specialize undef.

Also, there was a test that didn't work, so we normalized it.