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/test/request_license_test.cpp

@@ -3755,7 +3755,7 @@ TEST_F(WvCdmRequestLicenseTest, UsageInfo_ReleaseThreeRecords) {
       decryptor_->GetUsageInfo(app_id, kProviderSessionTokenStreamingClip4,
                                kDefaultCdmIdentifier, &usage_info);
   EXPECT_EQ(KEY_MESSAGE, status);
-  EXPECT_EQ(1, usage_info.size());
+  ASSERT_EQ(1, usage_info.size());
   release_msg = GetUsageInfoResponse(config_.license_server(),
                                      config_.client_auth(), usage_info[0]);
   EXPECT_EQ(NO_ERROR,