find-assets handles get-resource returning nilern

- this can/will happen if a user doesn't have a resource root directory,
  e.g. if the resources/templates/asc/posts directory doesn't exist.

src/cryogen_core/io.clj

@@ -22,11 +22,18 @@
           matches (map #(re-find % name) ignored-files)]
       (not (some seq matches)))))
 
-(defn find-assets [f ext ignored-files]
+(defn find-assets
+  "Find all assets in the given root directory (f) and the given file
+  extension (ext) ignoring any files that match the given (ignored-files).
+  First make sure that the root directory exists, if yes: process as normal;
+  if no, return empty vector."
+  [f ext ignored-files]
+  (if-let [root (get-resource f)]
     (->> (get-resource f)
          file-seq
          (filter (ignore ignored-files))
-       (filter (fn [file] (-> file .getName (.endsWith ext))))))
+         (filter (fn [file] (-> file .getName (.endsWith ext)))))
+    []))
 
 (defn create-folder [folder]
   (let [loc (io/file (str public folder))]