Fixes issue boot-clj#555

This modifies the file-filter function so that if the criteria seq is
empty, it simply returns an empty list.

boot/core/src/boot/core.clj

@@ -1218,10 +1218,12 @@
   "A file filtering function factory. FIXME: more documenting here."
   [mkpred]
   (fn [criteria files & [negate?]]
-    (let [tmp?   (partial satisfies? tmpd/ITmpFile)
-          ->file #(if (tmp? %) (io/file (tmp-path %)) (io/file %))
-          pred (apply juxt (mapv mkpred criteria))]
-      (filter-files #(pred (->file %)) files negate?))))
+    (if (empty? criteria)
+      '()
+      (let [tmp?   (partial satisfies? tmpd/ITmpFile)
+            ->file #(if (tmp? %) (io/file (tmp-path %)) (io/file %))
+            pred (apply juxt (mapv mkpred criteria))]
+        (filter-files #(pred (->file %)) files negate?)))))
 
 (defn by-meta
   "This function takes two arguments: `preds` and `files`, where `preds` is a