Fix NPE when GCE region is empty

When GCE region is empty we get back from the API something like:

```
{
  "id": "dummy"
}
```

instead of:

```
{
  "id": "dummy",
  "items":[ ]
}
```

This generates a NPE when we aggregate all the lists into a single one.

Closes #16967.

plugins/discovery-gce/src/main/java/org/elasticsearch/cloud/gce/GceComputeServiceImpl.java

@@ -97,7 +97,7 @@ public class GceComputeServiceImpl extends AbstractLifecycleComponent<GceCompute
                     }
                 });
                 // assist type inference
-                return instanceList.isEmpty() ? Collections.<Instance>emptyList() : instanceList.getItems();
+                return instanceList.isEmpty()  || instanceList.getItems() == null ? Collections.<Instance>emptyList() : instanceList.getItems();
             } catch (PrivilegedActionException e) {
                 logger.warn("Problem fetching instance list for zone {}", e, zoneId);
                 logger.debug("Full exception:", e);

plugins/discovery-gce/src/test/java/org/elasticsearch/discovery/gce/GceDiscoveryTests.java

@@ -254,4 +254,17 @@ public class GceDiscoveryTests extends ESTestCase {
             assertThat(expected.getMessage(), containsString("one or more gce discovery settings are missing."));
         }
     }
+
+    /**
+     * For issue https://github.com/elastic/elasticsearch/issues/16967
+     */
+    public void testEmptyRegion16967() {
+        Settings nodeSettings = Settings.builder()
+            .put(GceComputeService.PROJECT_SETTING.getKey(), projectName)
+            .putArray(GceComputeService.ZONE_SETTING.getKey(), "europe-west1-b", "us-central1-a")
+            .build();
+        mock = new GceComputeServiceMock(nodeSettings, networkService);
+        List<DiscoveryNode> discoveryNodes = buildDynamicNodes(mock, nodeSettings);
+        assertThat(discoveryNodes, hasSize(1));
+    }
 }

plugins/discovery-gce/src/test/resources/org/elasticsearch/discovery/gce/compute/v1/projects/emptyregion16967/zones/europe-west1-b/instances

@@ -0,0 +1,36 @@
+{
+  "id": "dummy",
+  "items":[
+    {
+      "description": "ES Node 1",
+      "id": "9309873766428965105",
+      "kind": "compute#instance",
+      "machineType": "n1-standard-1",
+      "name": "test1",
+      "networkInterfaces": [
+        {
+          "accessConfigs": [
+            {
+              "kind": "compute#accessConfig",
+              "name": "External NAT",
+              "natIP": "104.155.13.147",
+              "type": "ONE_TO_ONE_NAT"
+            }
+          ],
+          "name": "nic0",
+          "network": "default",
+          "networkIP": "10.240.79.59"
+        }
+      ],
+      "status": "RUNNING",
+      "tags": {
+        "fingerprint": "xA6QJb-rGtg=",
+        "items": [
+          "elasticsearch",
+          "dev"
+        ]
+      },
+      "zone": "europe-west1-b"
+    }
+  ]
+}

plugins/discovery-gce/src/test/resources/org/elasticsearch/discovery/gce/compute/v1/projects/emptyregion16967/zones/us-central1-a/instances

@@ -0,0 +1,3 @@
+{
+  "id": "dummy"
+}