fix: add support for DATA_TYPE_NONE to om_variable_write_scalar

[terraputix]

This would allow to be able to simply create groups of variables.

However, maybe it is preferable to make a separate function om_variable_write_group to have a better logical separation from the scalar methods, e.g. it might be confusing that a None-type variable cannot be read with om_variable_read_scalar.
In this case, I would suggest to also remove DATA_TYPE_NONE from the om_variable_write_scalar_size function.