fix($http): apply transformResponse even when data is empty

Note, that (as a by-product of the previous implementation) only non-empty
data was passed through the `transformResponse` pipeline. This is no
longer the case.

When using a custom `transformResponse` function, one should make sure it
can also handle an empty (i.e. falsy) `data` argument appropriately.

Fixes #12976
Closes #12979

Author: gkalpak

src/ng/http.js

@@ -999,11 +999,8 @@ function $HttpProvider() {
       function transformResponse(response) {
         // make a copy since the response must be cacheable
         var resp = extend({}, response);
-        if (!response.data) {
-          resp.data = response.data;
-        } else {
-          resp.data = transformData(response.data, response.headers, response.status, config.transformResponse);
-        }
+        resp.data = transformData(response.data, response.headers, response.status,
+                                  config.transformResponse);
         return (isSuccess(response.status))
           ? resp
           : $q.reject(resp);

test/ng/httpSpec.js

@@ -1391,6 +1391,25 @@ describe('$http', function() {
           expect(callback).toHaveBeenCalledOnce();
           expect(callback.mostRecentCall.args[0]).toBe('RESP-FIRST:V1');
         });
+
+
+        it('should apply `transformResponse` even if the response data is empty', function(data) {
+          var callback = jasmine.createSpy('transformResponse');
+          var config = {transformResponse: callback};
+
+          $httpBackend.expect('GET', '/url1').respond(200, undefined);
+          $httpBackend.expect('GET', '/url2').respond(200, null);
+          $httpBackend.expect('GET', '/url3').respond(200, '');
+          $http.get('/url1', config);
+          $http.get('/url2', config);
+          $http.get('/url3', config);
+          $httpBackend.flush();
+
+          expect(callback.callCount).toBe(3);
+          expect(callback.calls[0].args[0]).toBe(undefined);
+          expect(callback.calls[1].args[0]).toBe(null);
+          expect(callback.calls[2].args[0]).toBe('');
+        });
       });
     });