Avoid NPE when accessing to EnforcedAdmin

NPE happens when "modify" is tapped on the carrier wifi network
connected using CarrierConfig information. null is passed to
sendShowAdminSupportDetailsIntent method through 2nd argument.

Bug: 128188146
Test: Manually verified the crash scenario
Test: RunSettingsLibRoboTests
Signed-off-by: Daichi Ueura <daichi.ueura@sony.com>
(Cherry-pick from pa/1302748)
Change-Id: If92c12041b91dbab0f083a1b49d263b201e87699