fix($compile): ignore optional =-bound properties with empty value

Previously, optional empty '='-bound expressions were ignored ---
erroneously they stopped being ignored, and no tests were caused to
fail for this reason. This change restores the original ignoring
behaviour while still preventing other errors fixed by 8a1eb16

Closes angular#12144
Closes angular#12259

Author: caitp

src/ng/compile.js

@@ -2588,9 +2588,9 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
 
           case '=':
             if (!hasOwnProperty.call(attrs, attrName)) {
-              if (optional) break;
               attrs[attrName] = void 0;
             }
+            if (optional && !attrs[attrName]) break;
 
             parentGet = $parse(attrs[attrName]);
             if (parentGet.literal) {

test/ng/compileSpec.js

@@ -3618,6 +3618,33 @@ describe('$compile', function() {
     });
 
 
+    it('should ignore optional "="-bound property if value is the emptry string', function() {
+      module(function($compileProvider) {
+        $compileProvider.directive('testDir', valueFn({
+          scope: {prop: '=?'},
+          controller: function($scope) {
+            $scope.prop = $scope.prop || 'default';
+            this.getProp = function() {
+              return $scope.prop;
+            };
+          },
+          controllerAs: 'ctrl',
+          template: '<p></p>'
+        }));
+      });
+      inject(function($compile, $rootScope) {
+        element = $compile('<div test-dir></div>')($rootScope);
+        var scope = element.isolateScope();
+        expect(scope.ctrl.getProp()).toBe('default');
+        $rootScope.$digest();
+        expect(scope.ctrl.getProp()).toBe('default');
+        scope.prop = 'foop';
+        $rootScope.$digest();
+        expect(scope.ctrl.getProp()).toBe('foop');
+      });
+    });
+
+
     describe('bind-once', function() {
 
       function countWatches(scope) {