Fix misleading code in sendCredentialsOnUnlockIfRequired()

lockScreenSecretAvailable() takes a @NonNull credential, so the code in
sendCredentialsOnUnlockIfRequired() that conditionally passes a null
credential is incorrect.  This doesn't actually matter, since
sendCredentialsOnUnlockIfRequired() is never called with
CREDENTIAL_TYPE_NONE anyway.  But make it do the more logical thing of
skipping the call to lockScreenSecretAvailable() in this case.

Test: atest com.android.server.locksettings
Change-Id: I026f72ccaadef94a2f43cb6c2a70e8c08d21360c