Output format: Which properties (if any) can be null omitted?

For a valid header block, can any of the `text` or `level` properties under any circumstances (now or in the future) be `null` or omitted? Or are any of the aforementioned properties guaranteed to always be present and non-`null` for this block to be valid?

Context: I'm a back-end .NET developer needing to parse Editor.js documents. The readme lists the aforementioned properties but does not explicitly guarantee that they will always be present and non-null. Back-end deserialization/parsing in strongly typed languages would be easier with some guarantees about which properties can and can not be null/omitted. (Ideally, any such guarantees should be added to the readme to ensure they are formalized and kept in the future.)