Add CLI Option to Purge Empty Fields in ES

Currently we don't do any sort of checks to see if a field is non-empty before shipping off to ES. This can lead to a lot of empty fields being sent out, depending on the use case and the benchmark. For instance, here is a document from a Uperf CI test ran by ripsaw, where most of the fields are populated through environment variables:

```json
{
"_index" : "ripsaw-uperf-results-000002",
"_type" : "_doc",
"_id" : "297e5bb466870ad7f90916e68f60c440a43be6dbe0707ab6f3c4d7e30007e807",
"_score" : 7.654378,
"_source" : {
"workload" : "uperf",
"uuid" : "79690e49-479e-593a-8a51-0a1ef032de88",
"user" : "ripsaw",
"cluster_name" : "myk8scluster",
"hostnetwork" : "True",
"iteration" : 2,
"remote_ip" : "10.0.133.30",
"client_ips" : "10.0.173.62 10.130.0.1 ",
"uperf_ts" : "2021-06-01T22:32:39.594000",
"service_ip" : "False",
"bytes" : 519864320,
"norm_byte" : 258605056,
"ops" : 1015360,
"norm_ops" : 505088,
"norm_ltcy" : 2.3778479412250935,
"kind" : "pod",
"client_node" : "ip-10-0-173-62.us-west-2.compute.internal",
"server_node" : "unknown",
"num_pairs" : "1",
"multus_client" : "",
"networkpolicy" : "",
"density" : "1",
"nodes_in_iter" : "1",
"step_size" : "",
"colocate" : "False",
"density_range" : [ ],
"node_range" : [ ],
"pod_id" : "0",
"test_type" : "stream",
"protocol" : "udp",
"message_size" : 512,
"read_message_size" : 512,
"num_threads" : 2,
"duration" : 3,
"run_id" : "NA"
}
}
```

And here is a document exported from just running the command ``run_snafu --tool uperf --user ryan --uuid 1234 --proto tcp --remoteip localhost -w iperf.xml --resourcetype container -s 1 --verbose``:

```json
{
"_index": "snafu-uperf-results",
"_op_type": "create",
"_source": {
"test_type": "",
"protocol": "tcp",
"message_size": null,
"read_message_size": null,
"num_threads": 1,
"duration": 31,
"kind": "container",
"hostnetwork": "False",
"remote_ip": "localhost",
"client_ips": "",
"service_ip": "False",
"client_node": "",
"server_node": "",
"num_pairs": "",
"multus_client": "",
"networkpolicy": "",
"density": "",
"nodes_in_iter": "",
"step_size": "",
"colocate": "",
"density_range": "",
"node_range": "",
"pod_id": null,
"uperf_ts": "2021-06-28T14:42:37.066000",
"bytes": 48083435520,
"norm_byte": 1505771520,
"ops": 5869560,
"norm_ops": 183810,
"norm_ltcy": 6.534098878427453,
"iteration": 1,
"user": "ryan",
"uuid": "1234",
"workload": "uperf",
"run_id": "NA"
},
"_id": "ae6d9dfc7083e94c569d1999c2eb2ae1dce4a77fc5c7052c128103783f9acc70",
"run_id": "NA"
}
```

I think it would be cool to add in a CLI option called ``--no-empty-fields`` or something, that would remove any field from exported documents which is null or an empty string. This way teams only get the fields and the data that they care about, rather than also getting the extra fields which we use as a team.

Start by reproducing the shown run_snafu command and trace where its exported document is assembled before being sent to Elasticsearch. Done means a --no-empty-fields option is accepted and removes null or empty-string fields from exported documents while retaining populated fields.

Written by the indexing model from the issue text.