Write empty values (null and no children) as an object when saving to binary

[LossyDragon]

Description

Checks off 937 in #181

This will match the behaviour of text saving. And looking at Valve's code, they default to type none, so when serializing it should be saved as an object as well.

I believe right now deserializing an empty object, and then serializing back into binary would not be roundtrip in Steamkit.

Checklist

• Code compiles correctly
• All tests passing
• Samples run successfully
• Extended the README / documentation, if necessary