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 https://github.com/sass/node-sass/issues/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);
         }
       };
     }

test/api.js

@@ -325,6 +325,17 @@ describe('api', function() {
       });
     });
 
+    it('should fallback to default import behaviour if importer returns sass.NULL', function(done) {
+      sass.render({
+        file: fixture('include-files/index.scss'),
+        importer: function(url, prev, done) {
+          done(sass.NULL);
+        }
+      }, function(error, result) {
+        assert.equal(result.css.toString().trim(), '/* foo.scss */\n/* bar.scss */');
+        done();
+      });
+    });
 
     it('should fallback to default import behaviour if importer returns null for backwards compatibility', function(done) {
       sass.render({
@@ -1010,7 +1021,7 @@ describe('api', function() {
             return sass.types.String('foo');
           },
           bar: function(a) {
-            assert.strictEqual(a, sass.NULL, 
+            assert.strictEqual(a, sass.NULL,
                 'Supplied value should be the same instance as sass.NULL');
 
             assert.throws(function() {