Fix kotlin nullable errors in CaptivePortalLogin tests am: d2d0976b02 am: e234dffceb

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

Change-Id: Ie589a7b1041686557974225b3cd1b5c055ada3a6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>