fix(material/datepicker): pass correct inject flags to injector (#22665)

We have a call to `Injector.get` in the date range input which is supposed to fall back to `null` if the value doesn't resolve, however we're missing the `Optional` flag. It works by accident at the moment, because the inject flags aren't actually being passed through by the framework.

angular/angular#41592 is supposed to fix the framework issue, but their CI will break due to our usage.

These changes fix our usage in order to unblock the framework PR.

Author: crisbeto

src/material/datepicker/date-range-input-parts.ts

@@ -102,7 +102,8 @@ abstract class MatDateRangeInputPartBase<D>
     // itself. Usually we can work around it for the CVA, but there's no API to do it for the
     // validator. We work around it here by injecting the `NgControl` in `ngOnInit`, after
     // everything has been resolved.
-    const ngControl = this._injector.get(NgControl, null, InjectFlags.Self);
+    // tslint:disable-next-line:no-bitwise
+    const ngControl = this._injector.get(NgControl, null, InjectFlags.Self | InjectFlags.Optional);
 
     if (ngControl) {
       this.ngControl = ngControl;