Switch else case to null in when expression to make it exhaustive

Using else to make the when expression exhaustive won't flag future
enum values that are not handled, so use the explicit null case
instead.

Test: m PackageManagerServiceHostTests
Change-Id: I818587d181231bbc198b731637248ed18bbcac21

services/tests/PackageManagerServiceTests/host/src/com/android/server/pm/test/SystemStubMultiUserDisableUninstallTest.kt

@@ -628,7 +628,7 @@ class SystemStubMultiUserDisableUninstallTest : BaseHostJUnit4Test() {
             CodePath.SAME, CodePath.DIFFERENT ->
                 throw AssertionError("secondDataPath cannot be a data path")
             CodePath.SYSTEM -> assertThat(codePaths[1]).isEqualTo(stubFile.parent.toString())
-            else -> {}
+            null -> {}
         }
     }