Workshop itself does not handle this. Teachers are supposed to check all the aggregated grades before they close the workshop. I usually do it by sorting the grades report by Submission grades and then by the Assessment grade to see the best and the worst cases.
If you spot there is a student with no grade aggregated, you as the teacher have to go and provide the assessment yourself (there is a button "Assess" displayed for you just below any submission). Once you grade the submission, do not forget to re-calculate grades so that is taken into account.
Once you are happy with all the grades at the end of the Grading evaluation phase, close the workshop and both grades will be written to the course gradebook and received assessments are made available to the authors.