fix(animations): ensure void => * animations are triggered when an expression is omitted

Closes #9327
Closes #9381

modules/@angular/compiler/src/template_parser.ts

@@ -562,6 +562,12 @@ class TemplateParseVisitor implements HtmlAstVisitor {
   private _parseAnimation(
       name: string, expression: string, sourceSpan: ParseSourceSpan,
       targetMatchableAttrs: string[][], targetAnimationProps: BoundElementPropertyAst[]) {
+    // This will occur when a @trigger is not paired with an expression.
+    // For animations it is valid to not have an expression since */void
+    // states will be applied by angular when the element is attached/detached
+    if (!isPresent(expression) || expression.length == 0) {
+      expression = 'null';
+    }
     var ast = this._parseBinding(expression, sourceSpan);
     targetMatchableAttrs.push([name, ast.source]);
     targetAnimationProps.push(new BoundElementPropertyAst(

modules/@angular/core/test/animation/animation_integration_spec.ts

@@ -150,6 +150,49 @@ function declareTests({useJit}: {useJit: boolean}) {
                    });
              })));
 
+      it('should animate between * and void and back even when no expression is assigned',
+         inject(
+             [TestComponentBuilder, AnimationDriver],
+             fakeAsync((tcb: TestComponentBuilder, driver: MockAnimationDriver) => {
+               tcb = tcb.overrideTemplate(DummyIfCmp, `
+              <div @myAnimation *ngIf="exp"></div>
+            `);
+               tcb.overrideAnimations(DummyIfCmp, [trigger(
+                                                      'myAnimation',
+                                                      [
+                                                        state('*', style({'opacity': '1'})),
+                                                        state('void', style({'opacity': '0'})),
+                                                        transition('* => *', [animate('500ms')])
+                                                      ])])
+                   .createAsync(DummyIfCmp)
+                   .then((fixture) => {
+                     tick();
+
+                     var cmp = fixture.debugElement.componentInstance;
+                     cmp.exp = true;
+                     fixture.detectChanges();
+                     flushMicrotasks();
+
+                     var result = driver.log.pop();
+                     expect(result['duration']).toEqual(500);
+                     expect(result['startingStyles']).toEqual({'opacity': '0'});
+                     expect(result['keyframeLookup']).toEqual([
+                       [0, {'opacity': '0'}], [1, {'opacity': '1'}]
+                     ]);
+
+                     cmp.exp = false;
+                     fixture.detectChanges();
+                     flushMicrotasks();
+
+                     result = driver.log.pop();
+                     expect(result['duration']).toEqual(500);
+                     expect(result['startingStyles']).toEqual({'opacity': '1'});
+                     expect(result['keyframeLookup']).toEqual([
+                       [0, {'opacity': '1'}], [1, {'opacity': '0'}]
+                     ]);
+                   });
+             })));
+
       it('should combine repeated style steps into a single step',
          inject(
              [TestComponentBuilder, AnimationDriver],