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.

https://partnerissuetracker.corp.google.com/issues/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
diff --git a/hostsidetests/deviceidle/src/com/android/cts/deviceidle/DeviceIdleWhitelistTest.java b/hostsidetests/deviceidle/src/com/android/cts/deviceidle/DeviceIdleWhitelistTest.java
index bdba196..def9d75 100644
--- a/hostsidetests/deviceidle/src/com/android/cts/deviceidle/DeviceIdleWhitelistTest.java
+++ b/hostsidetests/deviceidle/src/com/android/cts/deviceidle/DeviceIdleWhitelistTest.java
@@ -91,7 +91,9 @@
         final List<String> packages = new ArrayList<>();
         for (String line : output.split("\n")) {
             final int i = line.indexOf(',');
-            packages.add(line.substring(0, i));
+            if (i > 0) {
+                packages.add(line.substring(0, i));
+            }
         }
         return packages;
     }