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.

Signed-off-by: James Clark <james.clark@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>