PrepareAVWorkflowOperationHandler.start() can throw unchecked NPE

PrepareAVWorkflowOperationHandler.start() will throw an unchecked exception on the logging line:

logger.debug("Running a/v muxing workflow operation on workflow {}", workflowInstance.getId());

If the workflow instance passed in is null. The logging line should be moved into the try/catch.

No test has been entered for this bug yet.