[FEAT] Handle batches with partial results

[MatthieuMoreau0]

• Stop raising an Exception if one of the job of the batch has no results after fetching.
• Stop raising an exception when fetching results if the status of the batch is a termination status.

Currently if one of the job of a batch fails, it is impossible to retrieve the results for any jobs of the batch - this raises an exception. In this PR, I suggest instead to add None to the list of results for that submission when a job has no results. I don't know if returning None is the most idiomatic way to do this in pulser; I also contemplated creating a new subclass of Result called EmptyResult but I wanted to gather some feedback before going for that because it would imply some refactoring of the Result class and I'm not entirely convinced it would bring much.