Fix kotlin nullable errors in CaptivePortalLogin tests am: d2d0976b02 am: e234dffceb am: 12cf9f64dc am: 3780dfc024

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

Change-Id: I2d0ff1ea504cbff4ca0d1134a9585c39b0644dca
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>