fix(select) aria-labelledby (#7022)

aria-labelledby must not be in the DOM if it is empty
this causes accessability bugs.

Author: Lindsor

src/lib/select/select.spec.ts

@@ -1802,6 +1802,15 @@ describe('MatSelect', () => {
         expect(select.getAttribute('aria-labelledby')).toBe('myLabelId');
       });
 
+      it('should not have aria-labelledby in the DOM if it`s not specified', async(() => {
+
+        fixture.detectChanges();
+
+        fixture.whenStable().then(() => {
+          expect(select.hasAttribute('aria-labelledby')).toBeFalsy();
+        });
+      }));
+
       it('should set the tabindex of the select to 0 by default', () => {
         expect(select.getAttribute('tabindex')).toEqual('0');
       });

src/lib/select/select.ts

@@ -371,7 +371,7 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
   @Input('aria-label') ariaLabel: string = '';
 
   /** Input that can be used to specify the `aria-labelledby` attribute. */
-  @Input('aria-labelledby') ariaLabelledby: string = '';
+  @Input('aria-labelledby') ariaLabelledby: string;
 
   /** Unique id of the element. */
   @Input()