ES|QL: fix generative tests

x-pack/plugin/esql/qa/server/single-node/src/javaRestTest/java/org/elasticsearch/xpack/esql/qa/single_node/GenerativeIT.java

@@ -34,4 +34,9 @@ public class GenerativeIT extends GenerativeRestTest {
     protected String getTestRestCluster() {
         return cluster.getHttpAddresses();
     }
+
+    @Override
+    protected boolean supportsSourceFieldMapping() {
+        return cluster.getNumNodes() == 1;
+    }
 }

x-pack/plugin/esql/qa/server/src/main/java/org/elasticsearch/xpack/esql/qa/rest/generative/EsqlQueryGenerator.java

@@ -164,7 +164,7 @@ private static String grok(List<Column> previousOutput) {
             } else {
                 String fieldName = randomRawName(previousOutput);
                 if (fieldName.isEmpty()) { // it's a bug, managed later, skipping for now
-                    randomAlphaOfLength(5);
+                    fieldName = randomAlphaOfLength(5);
                 }
                 result.append(fieldName);
             }

x-pack/plugin/esql/qa/server/src/main/java/org/elasticsearch/xpack/esql/qa/rest/generative/GenerativeRestTest.java

@@ -67,10 +67,12 @@ public abstract class GenerativeRestTest extends ESRestTestCase {
     @Before
     public void setup() throws IOException {
         if (indexExists(CSV_DATASET_MAP.keySet().iterator().next()) == false) {
-            loadDataSetIntoEs(client(), true, true);
+            loadDataSetIntoEs(client(), true, supportsSourceFieldMapping());
         }
     }
 
+    protected abstract boolean supportsSourceFieldMapping();
+
     @AfterClass
     public static void wipeTestData() throws IOException {
         try {