Handle empty backtrace from Solid Queue failed execution

This happens if we have to mark a job as failed because the worker
processing it died. There won't be a proper backtrace in the exception
in that case.

lib/active_job/queue_adapters/solid_queue_ext.rb

@@ -118,7 +118,7 @@ def execution_error_from_solid_queue_job(solid_queue_job)
         ActiveJob::ExecutionError.new \
           error_class: solid_queue_job.failed_execution.exception_class,
           message: solid_queue_job.failed_execution.message,
-          backtrace: solid_queue_job.failed_execution.backtrace
+          backtrace: solid_queue_job.failed_execution.backtrace || []
       end
     end