perf report: Fix -T/--threads option to work again

The commit 512ae1bd

 ("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.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1430309328-28317-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>