use a blank string for empty body instead of nil

otherwise the string "null" ends up used as the response body

src/clj_webtoolbox/response.clj

@@ -9,7 +9,7 @@
 (def ^:private base-response
   {:status  200
    :headers {}
-   :body    nil})
+   :body    ""})
 
 (defn content
   "Returns a Ring response where the body is set to the value given."

src/clj_webtoolbox/response_helpers.clj

@@ -3,7 +3,7 @@
     [clj-webtoolbox.response :as response]))
 
 (defn- ->response [body status]
-  (-> (response/content body)
+  (-> (response/content (if (nil? body) "" body))
       (response/status status)))
 
 (defn ok [& [body]] (->response body 200))