Allow enforcingAdmin parameter to be nullable to address an NPE.

There are cases where this may need to be passed in null and should be handled gracefully.

Bug: 426094493
Test: atest ActionDisabledByAdminDialogHelperTest
Flag: android.app.supervision.flags.deprecate_dpm_supervision_apis
Change-Id: Ia7fc3cc108001ab23289b3182bdf34ee8ba4d616