Change `None` Serialization

[ambsw-technology]

When uploading to SSM None currently gets stringified to "None". This is just semantically wrong. Some options to resolve this include:

• Serialize as empty string
• Delete a key if set to None

Empty strings are ambiguous (and could be passed explicitly) so delete is probably the most semantically faithful storage strategy. Unfortunately, it could create issues for consumers who expect the key to exist and instead get an exception. Fortunately, SSM is mostly used for ENV variables and an empty string will normally unset the values so we don't lose much by going this route.