[FSSDK-8948] fix(odp): check odp identifiers not null or empty before sending

[mnoman09]

Summary

• Check if ODP events have at least one key-value pair for identifiers. If not, discard the request without sending to the ODP server.

Test plan

• Add a test with empty identifiers and null identifier.
• All tests should pass.

Issues

• FSSDK-8948

[mikechu-optimizely]

@mnoman09, This is ready to merge whenever you'd like.