fix: process all data on_output, even when not terminated with newline

The current condition prematurely quits processing of data in on_output
in some edge-cases.

In these cases `job:result()` will have all data, but not everyting was
sent through the `on_output` pipes.

Closes #634

Author: msvechla

lua/plenary/job.lua

@@ -361,7 +361,7 @@ local on_output = function(self, result_key, cb)
         cb(err, result_line, self)
       end
 
-      if data == nil or is_complete then
+      if is_complete then
         return
       end