Don't report metric for no escrow data

Since the RoR flow is called for every reboot, this metric captures when
RoR is not attempted which is an ineffective metric.

Bug: 362363906
Test: atest CtsResumeOnRebootHostTestCases \
FrameworksServicesTests: RebootEscrowDataTest \
FrameworksServicesTests: LockSettingsServiceTests \
FrameworksServicesTests: RecoverySystemServiceTest \
FrameworksServicesTests: RebootEscrowManagerTests \
FrameworksServicesTests: RebootEscrowProviderServerBasedImplTests
Change-Id: Ia701790e5768510791de2b3b78eb8063bca84046