[wpa_supplicant] Remove null checks for URL in deauth imminent

The URL in deauth imminent message is optional and can be null.
Remove all null checks of the URL to allow the framework to
process deauth imminent notifications with no reason URLs.

Bug: 171320371
Test: Compiles. Not in use (yet)
Change-Id: I916d1d08f3bab033c4cf5361d0c77bef5fac6d67
3 files changed