Of the 50 or so people who attended a somber candlelight vigil for Alyssa Byrne Thursday, no one knew her. But each was affected by the 19-year-old Petaluma resident who went missing following the final night of the SnowGlobe Music Festival and was found dead several days later.Many shivered through a crisp Thursday sunset in hopes of showing support for Byrne's family. Some said they hoped to help prevent a similar incident in the future.Of all the people in attendance Thursday, it is likely none was more affected by Byrne's death than Antonio Diaz, one of the South Tahoe Public …