add warning support for no body

Author: jakelandis

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

@@ -148,6 +148,11 @@ public Email.Builder render(TextTemplateEngine engine, Map<String, Object> model
             renderedHtml = htmlSanitizer.sanitize(renderedHtml);
             builder.htmlBody(renderedHtml);
         }
+
+        if(htmlBody == null && textBody == null && Strings.isNullOrEmpty(textWarnings) == false){
+            builder.textBody(textWarnings);
+        }
+
         return builder;
     }