HLRest: Fix test for explain API

The test could sometimes generate an empty array of random stored fields
to fetch which would cause the test to fail. It is fine not to allow
empty here because we already randomize whether or not to generate the
stored fields param anyway.

Author: nik9000

client/rest-high-level/src/test/java/org/elasticsearch/client/RequestConvertersTests.java

@@ -1442,7 +1442,7 @@ public void testExplain() throws IOException {
             expectedParams.put("preference", preference);
         }
         if (randomBoolean()) {
-            String[] storedFields = generateRandomStringArray(10, 5, false);
+            String[] storedFields = generateRandomStringArray(10, 5, false, false);
             String storedFieldsParams = randomFields(storedFields);
             explainRequest.storedFields(storedFields);
             expectedParams.put("stored_fields", storedFieldsParams);