[tests] SearchApiRuntimeFieldsTests can return null values

[Mpdreamz]

Failed collections:

• ReadOnly: Tests.Search.Search.SearchApiRuntimeFieldsTests
  ---Reproduce: -----
  ./build.sh seed:99540 random:sourceserializer random:typedkeys random:httpcompression:false integrate 8.0.0-SNAPSHOT "readonly" "searchapiruntimefields"

Sometimes the fields returned are null and our assertions are not setup to handle that.

ests.Framework.EndpointTests.ResponseAssertionException : Expected response.Hits.First().Fields.ValueOf<ProjectRuntimeFields, string>(p => p.StartedOnDayOfWeek) not to be <null> or empty, but found <null>.
Response Under Test:
Valid NEST response built from a successful (200) low level call on POST: /project/_search?pretty=true&error_trace=true&typed_keys=true
# Audit trail of this API call:
 - [1] HealthyResponse: Node: http://localhost:9200/ Took: 00:00:00.0296053
# Request:
{"fields":["runtime_started_on_day_of_week",{"field":"runtime_thirty_days_after_started","format":"basic_date"},"search_runtime_field"],"query":{"match_all":{}},"size":5,"runtime_mappings":{"search_runtime_field":{"script":{"lang":"painless","source":"if (doc['type'].size() != 0) {emit(doc['type'].value.toUpperCase())}"},"type":"keyword"}}}
# Response:

[stevejgordon]

Closed with #5462