Resolve "Changes to SessionDetailsDTO: Support for hybrid labs"

What does this MR do (summary)?

See #150 (closed) no additional desc needed.

Screenshots

What changes have you made (detailed)?

Changed @NotEmpty to @NotNull for 3 attributes.

Does this MR meet the acceptance criteria?

• The main application was run to make sure the server still runs without errors.
• Tests were written to verify the behaviour of this code.
• Javadoc and other comments were added to make the code understandable.
• The code adheres to the style guidelines.

Closes #150 (closed)