fix(job): shutdown raises error (#296)

`job:shutdown()` raises an error if called without a `_shutdown_check` running.
 From what I experienced that is basically always the case.
This just checks if `_shutdown_check` is nil before passing it to `vim.loop.is_active`

Author: saecki

lua/plenary/job.lua

@@ -214,7 +214,7 @@ end
 
 --- Shutdown a job.
 function Job:shutdown(code, signal)
-  if not uv.is_active(self._shutdown_check) then
+  if self._shutdown_check and not uv.is_active(self._shutdown_check) then
     vim.wait(1000, function()
       return self:_pipes_are_closed(self) and self.is_shutdown
     end, 1, true)