perf hists browser: replace rb_first() != NULL by !RB_EMPTY_ROOT()

Its way too stupid to use rb_first() for just caching if there are
children, use the cheaper RB_EMPTY_ROOT() instead.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>