Address the possibility of vectors being empty in vector/_first

The original version just used the built-in `first`, which returns `nil` if you give it an empty collection, which is almost certainly not a useful behavior.

This changes it to return `:ignore-instruction` if the vector is empty, thereby leaving all the stacks unchanged.

src/propeller/push/instructions/vector.cljc

@@ -52,12 +52,16 @@
     (make-instruction state empty? [stack] :boolean)))
 
 ;; Pushes the first item of the top element of the vector stack onto the
-;; approrpiately-typed literal stack
+;; approrpiately-typed literal stack. If the vector is empty, return
+;; :ignore-instruction so that nothing is changed on the stacks.
 (def _first
   ^{:stacks #{:elem}}
   (fn [stack state]
     (let [lit-stack (get-vector-literal-type stack)]
-      (make-instruction state first [stack] lit-stack))))
+      (make-instruction state
+                        #(if (empty? %) :ignore-instruction (first %))
+                        [stack] 
+                        lit-stack))))
 
 ;; Pushes onto the INTEGER stack the index of the top element of the
 ;; appropriately-typed literal stack within the top element of the vector stack