Fix events with null obs

The following kind of event causes a crash due to the null literal value in the obs.values when nothing is selected for the question and the org.smartregister.converters.EventConverter#convertEventToEncounterResource during plan evaluation
Family Member Registration Event
{
  "baseEntityId": "8dbc3015-abd8-401c-b662-274573f1ed92",
  "childLocationId": "Kenya",
  "duration": 0,
  "entityType": "ec_family_member",
  "eventDate": "2020-12-08T00:00:00.000Z",
  "eventType": "Family Member Registration",
  "formSubmissionId": "9616c316-05ee-437a-8e36-72389f9f2c75",
  "locationId": "ac7ba751-35e8-4b46-9e53-3cbaad193697",
  "obs": [
    {
      "fieldCode": "same_as_fam_name",
      "fieldDataType": "text",
      "fieldType": "formsubmissionField",
      "formSubmissionField": "same_as_fam_name",
      "humanReadableValues": [],
      "parentCode": "",
      "saveObsAsArray": false,
      "values": [
        "true"
      ]
    },
    {
      "fieldCode": "dob_unknown",
      "fieldDataType": "text",
      "fieldType": "formsubmissionField",
      "formSubmissionField": "dob_unknown",
      "humanReadableValues": [],
      "keyValPairs": {
        "dob_unknown": "DOB unknown?"
      },
      "parentCode": "",
      "saveObsAsArray": false,
      "values": [
        "DOB unknown?"
      ]
    },
    {
      "fieldCode": "1712AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
      "fieldDataType": "select one",
      "fieldType": "concept",
      "formSubmissionField": "highest_edu_level",
      "humanReadableValues": [
        "Highest education level"
      ],
      "parentCode": "",
      "saveObsAsArray": false,
      "values": [
        null
      ]
    },
    {
      "fieldCode": "last_interacted_with",
      "fieldDataType": "text",
      "fieldType": "formsubmissionField",
      "formSubmissionField": "last_interacted_with",
      "humanReadableValues": [],
      "parentCode": "",
      "saveObsAsArray": false,
      "values": [
        "1607422593965"
      ]
    },
    {
      "fieldCode": "163137AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
      "fieldDataType": "start",
      "fieldType": "concept",
      "formSubmissionField": "start",
      "humanReadableValues": [],
      "parentCode": "",
      "saveObsAsArray": false,
      "values": [
        "2020-12-08 13:15:21"
      ]
    },
    {
      "fieldCode": "163138AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
      "fieldDataType": "end",
      "fieldType": "concept",
      "formSubmissionField": "end",
      "humanReadableValues": [],
      "parentCode": "",
      "saveObsAsArray": false,
      "values": [
        "2020-12-08 13:16:33"
      ]
    },
    {
      "fieldCode": "163149AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
      "fieldDataType": "deviceid",
      "fieldType": "concept",
      "formSubmissionField": "deviceid",
      "humanReadableValues": [],
      "parentCode": "",
      "saveObsAsArray": false,
      "values": [
        "358240051111110"
      ]
    },
    {
      "fieldCode": "163150AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
      "fieldDataType": "subscriberid",
      "fieldType": "concept",
      "formSubmissionField": "subscriberid",
      "humanReadableValues": [],
      "parentCode": "",
      "saveObsAsArray": false,
      "values": [
        "310260000000000"
      ]
    },
    {
      "fieldCode": "163151AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
      "fieldDataType": "simserial",
      "fieldType": "concept",
      "formSubmissionField": "simserial",
      "humanReadableValues": [],
      "parentCode": "",
      "saveObsAsArray": false,
      "values": [
        "89014103211118510720"
      ]
    },
    {
      "fieldCode": "163152AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
      "fieldDataType": "phonenumber",
      "fieldType": "concept",
      "formSubmissionField": "phonenumber",
      "humanReadableValues": [],
      "parentCode": "",
      "saveObsAsArray": false,
      "values": [
        "+15555215554"
      ]
    }
  ],
  "providerId": "onatest",
  "team": "Dream Team Organisation",
  "teamId": "0c7fd90f-5b99-4ed9-b337-0b42d51eab97",
  "version": 1607422593972,
  "clientApplicationVersion": 1,
  "clientDatabaseVersion": 5,
  "dateCreated": "2020-12-08T13:16:33.972Z",
  "type": "Event",
  "details": {
    "planIdentifier": "d652d9ae-2ff9-41e4-9bb3-fd4cd2ea0e2e"
  }
}
opensrp / opensrp-client-goldsmith

Fix events with null obs #27
