Handle sass.NULL returned from the importer via done()

Whenever done() is called we should check for the sass.NULL
value as well.

While here, allow calling done(null) to anticipate
removal of the sass.NULL one day.

Fixes sass#1296

lib/index.js

@@ -301,27 +301,27 @@ module.exports.render = function(options, cb) {
     if (Array.isArray(importer)) {
       importer.forEach(function(subject, index) {
         options.importer[index] = function(file, prev, bridge) {
-          function done(data) {
-            bridge.success(data);
+          function done(result) {
+            bridge.success(result === module.exports.NULL ? null : result);
           }
 
           var result = subject.call(options.context, file, prev, done);
 
-          if (result) {
-            done(result === module.exports.NULL ? null : result);
+          if (result !== undefined) {
+            done(result);
           }
         };
       });
     } else {
       options.importer = function(file, prev, bridge) {
-        function done(data) {
-          bridge.success(data);
+        function done(result) {
+          bridge.success(result === module.exports.NULL ? null : result);
         }
 
         var result = importer.call(options.context, file, prev, done);
 
-        if (result) {
-          done(result === module.exports.NULL ? null : result);
+        if (result !== undefined) {
+          done(result);
         }
       };
     }