change template expansion missing value handling (#36679) (#36715)

(cherry picked from commit 310d3ebe3d)

Co-authored-by: David Parrott <stomp.box.yo@gmail.com>

pkg/services/ngalert/state/cache.go

@@ -123,7 +123,7 @@ func expandTemplate(name, text string, data map[string]string) (result string, r
 		}
 	}()
 
-	tmpl, err := text_template.New(name).Option("missingkey=zero").Parse(text)
+	tmpl, err := text_template.New(name).Option("missingkey=error").Parse(text)
 	if err != nil {
 		return "", fmt.Errorf("error parsing template %v: %s", name, err.Error())
 	}