[ML] Improve error message when post-calendar-events-request is empty (elastic/x-pack-elasticsearch#3572)

Relates elastic/x-pack-elasticsearch#3016

Original commit: elastic/x-pack-elasticsearch@3d25ed4723

plugin/core/src/main/java/org/elasticsearch/xpack/ml/action/PostCalendarEventsAction.java

@@ -82,6 +82,10 @@ public class PostCalendarEventsAction extends Action<PostCalendarEventsAction.Re
         public Request(String calendarId, List<ScheduledEvent> scheduledEvents) {
             this.calendarId = ExceptionsHelper.requireNonNull(calendarId, Calendar.ID.getPreferredName());
             this.scheduledEvents = ExceptionsHelper.requireNonNull(scheduledEvents, EVENTS.getPreferredName());
+
+            if (scheduledEvents.isEmpty()) {
+                throw ExceptionsHelper.badRequestException("at least 1 event is required");
+            }
         }
 
         public String getCalendarId() {

plugin/src/test/resources/rest-api-spec/test/ml/calendar_crud.yml

@@ -584,3 +584,15 @@
   - length: { events: 2 }
   - match: { events.0.description: ski }
   - match: { events.1.description: snow }
+
+---
+"Test post calendar events given empty events":
+
+  - do:
+      catch: /at least 1 event is required/
+      xpack.ml.post_calendar_events:
+        calendar_id: "foo"
+        body: >
+          {
+            "events": []
+          }

qa/smoke-test-ml-with-security/build.gradle

@@ -22,6 +22,7 @@ integTestRunner {
     // know whether to expect an authorization exception or a validation exception
     'ml/calendar_crud/Test cannot create calendar with name _all',
     'ml/calendar_crud/Test PageParams with ID is invalid',
+    'ml/calendar_crud/Test post calendar events given empty events',
     'ml/custom_all_field/Test querying custom all field',
     'ml/datafeeds_crud/Test delete datafeed with missing id',
     'ml/datafeeds_crud/Test put datafeed referring to missing job_id',