fix(select): emitting change event twice for reset values

Fixes `mat-select` emitting its change event twice when a reset value is selected, as well as when it's selected twice in a row. This PR covers #10859 which would've introduced another issue.

Fixes #10675.
Fixes #13579.

Author: crisbeto

src/lib/select/select.spec.ts

@@ -2853,6 +2853,64 @@ describe('MatSelect', () => {
       expect(spy).toHaveBeenCalledWith('steak-0');
     }));
 
+    it('should emit once when a reset value is selected', fakeAsync(() => {
+      const fixture = TestBed.createComponent(BasicSelectWithoutForms);
+      const instance = fixture.componentInstance;
+      const spy = jasmine.createSpy('change spy');
+
+      instance.selectedFood = 'sandwich-2';
+      instance.foods[0].value = null;
+      fixture.detectChanges();
+
+      const subscription = instance.select.selectionChange.subscribe(spy);
+
+      fixture.debugElement.query(By.css('.mat-select-trigger')).nativeElement.click();
+      fixture.detectChanges();
+      flush();
+
+      (overlayContainerElement.querySelector('mat-option') as HTMLElement).click();
+      fixture.detectChanges();
+      flush();
+
+      expect(spy).toHaveBeenCalledTimes(1);
+
+      subscription.unsubscribe();
+    }));
+
+    it('should not emit the change event multiple times when a reset option is ' +
+      'selected twice in a row', fakeAsync(() => {
+        const fixture = TestBed.createComponent(BasicSelectWithoutForms);
+        const instance = fixture.componentInstance;
+        const spy = jasmine.createSpy('change spy');
+
+        instance.foods[0].value = null;
+        fixture.detectChanges();
+
+        const subscription = instance.select.selectionChange.subscribe(spy);
+
+        fixture.debugElement.query(By.css('.mat-select-trigger')).nativeElement.click();
+        fixture.detectChanges();
+        flush();
+
+        (overlayContainerElement.querySelector('mat-option') as HTMLElement).click();
+        fixture.detectChanges();
+        flush();
+
+        expect(spy).not.toHaveBeenCalled();
+
+        fixture.debugElement.query(By.css('.mat-select-trigger')).nativeElement.click();
+        fixture.detectChanges();
+        flush();
+
+        (overlayContainerElement.querySelector('mat-option') as HTMLElement).click();
+        fixture.detectChanges();
+        flush();
+
+        expect(spy).not.toHaveBeenCalled();
+
+        subscription.unsubscribe();
+      }));
+
   });
 
   describe('with option centering disabled', () => {

src/lib/select/select.ts

@@ -917,7 +917,10 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
     if (option.value == null && !this._multiple) {
       option.deselect();
       this._selectionModel.clear();
-      this._propagateChanges(option.value);
+
+      if (this.value != null) {
+        this._propagateChanges(option.value);
+      }
     } else {
       option.selected ? this._selectionModel.select(option) : this._selectionModel.deselect(option);