perf: test: Remove empty lines from branch filter test output

In the perf script command, spaces are turned into newlines. But when
there is a double space this results in empty lines which fail the
following inverse grep test, so strip the empty lines.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: linux-perf-users@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: James Clark <james.clark@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>