Handle empty results in CtsDeviceIdleHostTestCases

Tests gets syntax error when trying to split an empty list which
results from when cmd deviceidle sys-whitelist results is only
one package. Solved by returning before trying to split if string
is empty.

Bug: 155116999

Test: ran tc on devices with one or many whitelisted packages.

Signed-off-by: Gustav Svensson <gustav.svensson@volvo.corp-partner.google.com>
Change-Id: Ia13b6bb0df65c1cca7da1632fe9a2b7448125b40
(cherry picked from commit 2aa353e91b8f2c51733f7bc1650d2981bda9b710)