lisa.exekall_customize: Error for empty TestBundle.ftrace_conf['events']

There is no case where an FtraceCollector is requested to exekall by a TestBundle subclass without providing any events to collect. Therefore, it currently indicates a design issue, that would otherwise trigger a cryptic error in trace-cmd itself.

Such design issue can arise for aggregation+item class pairs, where exekall only knows about the aggregation class, but this class is an empty shell and does not provide a useful FtraceConf. In that case, the link must be manually made by adding ftrace_conf class attribute equal to the one from the item class.

Catch the issue when we can display relevant context and raise an exception.