Prevent buffering of job output and excessive polling (#4259)

When 'close_cb' is set for job_start(), but out_cb or err_cb isn't, vim
buffers data instead of dropping it (in case someone wanted to read and
process it in close_cb), and additionally polls for new data every 10
milliseconds, causing excessive wakeups and CPU usage. Since we don't
read the data anywhere outside of out_cb/err_cb, any LSP that prints an
error to stderr triggers this and vim keeps spinning until :ALEStopAllLSPs.

Fix this by always setting both callbacks, thus dropping any data we're
not interested in.

See vim/vim#10758 for an upstream report of
the excessive polling. It's possible this is intentional, I dunno.

Fixes: b42153e ("Fix #4098 - Clear LSP data when servers crash")

autoload/ale/job.vim

@@ -250,10 +250,16 @@ function! ale#job#Start(command, options) abort
 
         if has_key(a:options, 'out_cb')
             let l:job_options.out_cb = function('s:VimOutputCallback')
+        else
+            " prevent buffering of output and excessive polling in case close_cb is set
+            let l:job_options.out_cb = {->0}
         endif
 
         if has_key(a:options, 'err_cb')
             let l:job_options.err_cb = function('s:VimErrorCallback')
+        else
+            " prevent buffering of output and excessive polling in case close_cb is set
+            let l:job_options.err_cb = {->0}
         endif
 
         if has_key(a:options, 'exit_cb')