Fix kotlin nullable errors in CaptivePortalLogin tests am: d2d0976b02

Original change: https://android-review.googlesource.com/c/platform/packages/modules/CaptivePortalLogin/+/2700796

Change-Id: I4cebc8acaf5fb801d28d6449012ff25faa1586ed
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>