Fix migration query

Path can't non-null unless it's an import for an analysis job - which existing imports can't be

db/migrate/20240524011344_audio_event_import_files.rb

@@ -45,7 +45,7 @@ def change
         # create a default audio_event_import_file for each audio_event_import
         query = <<~SQL.squish
           INSERT INTO audio_event_import_files (audio_event_import_id, created_at, path, file_hash)
-          SELECT id, created_at, 'default', null FROM audio_event_imports;
+          SELECT id, created_at, null, null FROM audio_event_imports;
         SQL
 
         execute(query)