JI-5464 return null fields for empty state

[devland]

This will return null for empty saved states so that the h5p core library will no longer incorrectly show the "Your progress was restored" message.
Without this change the progress restored message is incorrectly shown on h5p.com instances when the user enters no data and refreshes the page.

[otacke]

I wish those things were documented somewhere instead of using H5P.com as some form of a hidden de-facto guideline that external contributors are not aware of.

I don't know about the internals of the restart button and what it considers to be a state that should enable the button: Should a state be returned at all if the user has not interacted with the page? Or should the state really contain null properties? Does the button really check all properties for being non-nullish?

I solved it in otacke/h5p-sort-paragraphs@03785a6 by not returning a state at all if the user has not answered the exercise.

[devland]

Should a state be returned at all if the user has not interacted with the page? Or should the state really contain null properties? Does the button really check all properties for being non-nullish?

The button checks the state by its attributes and it's considered to be empty if they are null or undefined.
false, empty strings or 0 are not considered to be empty attributes.

[otacke]

Thanks for letting me know. Your cleaned up version returns undefined instead of some state with "empty" parameters. Nice that way.