Appion

Looks like I did that on purpose; here is my comment from the code:
// NULL values should be added as empty, in case
// early iterations are NULL and later ones are set.
//if ( $newIterValue == NULL ) return;

I commented out the return on NULL to allow early iterations to be null with later iterations non-null.

Would you like me to add some code to remove parameters that only have null values after all iterations have been considered? Or is it better to leave it how it is to show that the parameter was not set for any of the iterations in the selected refinement?

I am not aware that we have cases of changing from null to not null at different iteration. Did one of the users request this?
It would be worth while checking what XMIPPS do in this case since this style is based on their syntax. I don't think python parser that we use can handle this. You will have to make python parser take default when it is written as "1x" if you want to keep this behavior.