Remove the empty property of attestation.

In GSI images attestation specific properties
(ro.product.*_for_attestation) are empty, which causes attestation
failure on devices with different values in ro.product.* and
ro.product.vendor.* from actual values provisioned in Keymint.

Hence these values added only if they are set in build files.

Bug:294190893
Test: atest CtsKeystoreTestCases:android.keystore.cts.DeviceOwnerKeyManagementTest#testAllVariationsOfDeviceIdAttestation

Change-Id: Icaf1bcd089767b2c47d0b57064aa72a4c9483bb5
Signed-off-by: wufei3 <wufei3@xiaomi.corp-partner.google.com>