Dont throw exception if admin is null

For policy_suspend_packages(or for other policies which are not user
restriction) admin component comes to be null always when
fetched from DPMS and the code changed as part of this change used to
throw exception. So handling this case to show generic learn more link
in policy transparency dialog which opens settings showing list of all
admins, if admin details are not found.

Bug: 203564700
Test: com.android.cts.devicepolicy.MixedProfileOwnerTest#testSuspendPackage
Change-Id: Ib3b4490eaa11ceab53f0004f4b7d089f3c0ba4fb