Fix delete- and update-by-query on indices without sequence numbers

[ywelsch]

This fixes a bug where delete-by-query and update-by-query were not working on an older index that did not have sequence numbers yet. The reason is that in a mixed-version 6.x/5.x cluster, documents can have a primary term but miss a sequence number.

After upgrade, this resulted in exceptions of the form:

{
    "error": {
        "root_cause": [
            {
                "type": "action_request_validation_exception",
                "reason": "Validation Failed: 1: ifSeqNo is unassigned, but primary term is [1];"
            }
        ],
        "type": "action_request_validation_exception",
        "reason": "Validation Failed: 1: ifSeqNo is unassigned, but primary term is [1];"
    },
    "status": 400
}

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Reindex)

[henningandersen]

LGTM.

[dnhatn]

LGTM. Thanks Yannick.