Don't use empty core message

Merge from Widevine repo of http://go/wvgerrit/93824

This treats an empty core message as not having a core message.

Bug: 149110740
Change-Id: Icacfc5d9a5bdce9b136c25c59205eee575cfba72
Tests: Ran ExoPlayer on taimen

libwvdrmengine/cdm/core/src/certificate_provisioning.cpp

@@ -360,11 +360,13 @@ CdmResponseType CertificateProvisioning::HandleProvisioningResponse(
   }
 
   if (supports_core_messages() &&
-      !signed_response.has_oemcrypto_core_message()) {
+      (!signed_response.has_oemcrypto_core_message() ||
+       signed_response.oemcrypto_core_message().empty())) {
     LOGE("Signed response does not have core message");
     error = true;
   } else if (!supports_core_messages() &&
-             signed_response.has_oemcrypto_core_message()) {
+             (signed_response.has_oemcrypto_core_message() &&
+              !signed_response.oemcrypto_core_message().empty())) {
     const std::string& core_message = signed_response.oemcrypto_core_message();
     // This case should not occur.  However, the CDM will let OEMCrypto
     // fail.