- Theme preview was not showing the same behaviour as a real theme because nulls were being encoded and decoded incorrectly causing nulls/empty strings to be treasted as truthy values
- Swap from using split to using proper query param parsing so that the code is more robust
- this still creates empty strings and the string 'null' so added a small function to decode these back to real nulls
- moved to its own file ready to be split out - there needs to be a bigger picture plan for this
- added unit tests to cover the known issues + some potential breakages from converting the header string to a query param object
