annotations: mini fix to event manager

public/app/features/annotations/event_manager.ts

@@ -58,7 +58,7 @@ export class EventManager {
   }
 
   addFlotEvents(annotations, flotOptions) {
-    if (this.event || annotations.length === 0) {
+    if (!this.event || annotations.length === 0) {
       return;
     }