fix($compile): returning null when an optional controller is not found

Currently, `undefined` is returned. However, the desired behavior is to
return `null` when the controller is optional and not found.

(If this breaks your app, it really shouldn't .v.)

Closes #9404
Closes #9392

src/ng/compile.js

@@ -1767,7 +1767,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
                 "Controller '{0}', required by directive '{1}', can't be found!",
                 require, directiveName);
           }
-          return value;
+          return value || null;
         } else if (isArray(require)) {
           value = [];
           forEach(require, function(require) {

test/ng/compileSpec.js

@@ -4197,38 +4197,38 @@ describe('$compile', function() {
     });
 
 
-    it("should not throw an error if required controller can't be found and is optional",function() {
+    it("should pass null if required controller can't be found and is optional",function() {
       module(function() {
         directive('dep', function(log) {
           return {
             require: '?^main',
             link: function(scope, element, attrs, controller) {
-              log('dep:' + !!controller);
+              log('dep:' + controller);
             }
           };
         });
       });
       inject(function(log, $compile, $rootScope) {
         $compile('<div main><div dep></div></div>')($rootScope);
-        expect(log).toEqual('dep:false');
+        expect(log).toEqual('dep:null');
       });
     });
 
 
-    it("should not throw an error if required controller can't be found and is optional with the question mark on the right",function() {
+    it("should pass null if required controller can't be found and is optional with the question mark on the right",function() {
       module(function() {
         directive('dep', function(log) {
           return {
             require: '^?main',
             link: function(scope, element, attrs, controller) {
-              log('dep:' + !!controller);
+              log('dep:' + controller);
             }
           };
         });
       });
       inject(function(log, $compile, $rootScope) {
         $compile('<div main><div dep></div></div>')($rootScope);
-        expect(log).toEqual('dep:false');
+        expect(log).toEqual('dep:null');
       });
     });