Allow transform macros to return null

It means they wish for there to be no output; which is pretty drastic
for a transform macro, but hey, user freedom.

Author: anko

src/translate.ls

@@ -19,7 +19,8 @@ module.exports = (root-env, ast, options={}) ->
   statements = ast.content
 
   transform-macros .for-each (macro) ->
-    statements := macro.apply null, statements
+    statements := (macro.apply null, statements)
+      .filter (isnt null)
 
   program-ast =
     type : \Program

test.ls

@@ -1027,6 +1027,11 @@ test "transform-macro can replace contents" ->
   esl "(+ 1 2)" transform-macros : [ wrapper ]
     .. `@equals` "3 * 4;"
 
+test "transform-macro can return null" ->
+  wrapper = -> null
+  esl "(+ 1 2)" transform-macros : [ wrapper ]
+    .. `@equals` ""
+
 test "transform-macro can receive arguments" ->
   wrapper = (...args) ->
     [ { atom : "hi" } ].concat args