Allow jitsi meetings

What does this MR do (summary)?

Screenshots

What changes have you made (detailed)?

Allow the list of rooms for a session to be empty, in case a session is held online.

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.