DTSW-7554-Update-the-handling-of-billboards-with-a-priority-of-zero

[liammcalpineduckietown]

This pull request refines the logic for selecting billboards from the local database in the get_billboard function. The update ensures that billboards with higher priorities are favored, and priority 0 billboards are only included if no higher-priority billboards exist. The selection process is now more robust and avoids errors if the database is empty or the selected billboard is missing.

Billboard selection logic improvements:

• Refactored the weighted selection to first determine the maximum priority among billboards, ensuring that only the highest-priority billboards are considered unless only priority 0 billboards exist.
• Improved error handling by checking for empty databases and missing billboard entries, returning None safely if no valid billboard is found.