The commit 512ae1bd6acb ("perf tools: Consolidate management of default
sort orders") changed default value of the 'sort_order' variable to NULL
indicating that users don't set any sort keys on the command line.
However it missed to update a check in perf_evlist__tty_browse_hists()
so that 'perf report -T' cannot show the per-thread values after the
normal output. This patch fixes it to work again.
Note that the -T option only works on --stdio and neither --sort nor
--parent option was given.