fix: Avoid crashing if not in cluster mode

[jcw-]

If PumaWorkerKiller is enabled, but Puma isn't in cluster mode (no workers), after roughly six hours, it crashes:

#<Thread:0x00007fdcbc998ef0 /Users/jcw/.rvm/gems/ruby-2.7.6/gems/puma_worker_killer-0.3.1/lib/puma_worker_killer/auto_reap.rb:14 run> terminated with exception (report_on_exception is true):
Traceback (most recent call last):
	4: from /Users/jcw/.rvm/gems/ruby-2.7.6/gems/puma_worker_killer-0.3.1/lib/puma_worker_killer/auto_reap.rb:17:in `block in start'
	3: from /Users/jcw/.rvm/gems/ruby-2.7.6/gems/puma_worker_killer-0.3.1/lib/puma_worker_killer/rolling_restart.rb:17:in `reap'
	2: from /Users/jcw/.rvm/gems/ruby-2.7.6/gems/puma_worker_killer-0.3.1/lib/puma_worker_killer/rolling_restart.rb:12:in `get_total_memory'
	1: from /Users/jcw/.rvm/gems/ruby-2.7.6/gems/puma_worker_killer-0.3.1/lib/puma_worker_killer/puma_memory.rb:53:in `get_total'
/Users/jcw/.rvm/gems/ruby-2.7.6/gems/puma_worker_killer-0.3.1/lib/puma_worker_killer/puma_memory.rb:75:in `set_workers': undefined method `each' for nil:NilClass (NoMethodError)

While it's true that this plugin doesn't make sense to run unless there's workers to monitor, in case someone does, this PR fixes the crash.