fix(ivy): use default selector for Components if selector is empty (#29239)

Prior to this change default selector for Components was not applied in case selector is missing or defined as an empty string. This update aligns this behavior between Ivy and VE: now default selector is used for Components when it's needed. Directives with empty selector are not allowed and trigger a compile-time error in both Ivy and VE.

PR Close #29239

packages/core/test/linker/integration_spec.ts

@@ -1426,6 +1426,20 @@ function declareTests(config?: {useJit: boolean}) {
             .toThrowError(`Directive ${stringify(SomeDirective)} has no selector, please add it!`);
       });
 
+      it('should throw when using directives with empty string selector', () => {
+        @Directive({selector: ''})
+        class SomeDirective {
+        }
+
+        @Component({selector: 'comp', template: ''})
+        class SomeComponent {
+        }
+
+        TestBed.configureTestingModule({declarations: [MyComp, SomeDirective, SomeComponent]});
+        expect(() => TestBed.createComponent(MyComp))
+            .toThrowError(`Directive ${stringify(SomeDirective)} has no selector, please add it!`);
+      });
+
       it('should use a default element name for components without selectors', () => {
         let noSelectorComponentFactory: ComponentFactory<SomeComponent> = undefined !;