add assert not null

Author: jakelandis

x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/notification/email/Attachment.java

@@ -42,6 +42,7 @@ protected Attachment(String id, String name, String contentType, boolean inline)
     protected Attachment(String id, String name, String contentType, boolean inline, Set<String> warnings) {
         super(id, name, contentType);
         this.inline = inline;
+        assert warnings != null;
         this.warnings = warnings;
     }