avoid self.targets is nil.

when two worker create `checker`, `worker-0` first created and add target success, then `worker-1` creating, when `worker-1` call `worker_events:poll()` in line 1408, raise crash.
`self.targets` init with empty table avoid this.

lib/resty/healthcheck.lua

@@ -1340,7 +1340,7 @@ function _M.new(opts)
   assert(self.shm, ("no shm found by name '%s'"):format(opts.shm_name))
 
   -- other properties
-  self.targets = nil     -- list of targets, initially loaded, maintained by events
+  self.targets = {}     -- list of targets, initially loaded, maintained by events
   self.events = nil      -- hash table with supported events (prevent magic strings)
   self.stopping = true   -- flag to indicate to timers to stop checking
   self.timer_count = 0   -- number of running timers