Resolve "NullPointerException: Cannot invoke "nl.tudelft.labracore.api.dto.StudentGroupDetailsDTO.getMemberUsernames()" because ..."

What does this mr do?

Actions taken to fix bug

Do a null check before trying to do operations on studentGroup

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made and the bug I fixed.
• A test was created to test the bug.
• I have updated the documentation accordingly.
• I adhere to the style guide.

Closes #543 (closed)