Fix NullPointerException in CaptivePortalLogin

[Problem]
After reboot, the CaptivePortalLogin crashes when start the
CaptivePortalLogin from Recents Screen.

[Root Cause]
Although Network class is null, Network#getPrivateDnsBypassingCopy is
called at CaptivePortalLoginActivity

[Solution]
To check if NetworkCapabilities is null before
Network#getPrivateDnsBypassingCopy

Bug: 113303484
Test: built, boot and manual

Change-Id: Icff8446fefa9700dcbcd1f8f1e4edeaf4b0d3557