fix(datepicker): update validation when switching from null to error

input

Author: mmalerba

src/lib/datepicker/datepicker-input.ts

@@ -329,6 +329,8 @@ export class MatDatepickerInput<D> implements ControlValueAccessor, OnDestroy, V
       this._cvaOnChange(date);
       this._valueChange.emit(date);
       this.dateInput.emit(new MatDatepickerInputEvent(this, this._elementRef.nativeElement));
+    } else {
+      this._validatorOnChange();
     }
   }