Adds a migration that casts the value of `ActivatedRouteSnapshot.fragment` to be non-nullable.
Also moves some code from the `AbstractControl.parent` migration so that it can be reused.
Relates to #37336.
PR Close#41092
As of #32671, the type of `AbstractControl.parent` can be null which can cause
compilation errors in existing apps. These changes add a migration that will append
non-null assertions to existing unsafe accesses.
````
// Before
console.log(control.parent.value);
// After
console.log(control.parent!.value);
```
The migration also tries its best to avoid cases where the non-null assertions aren't
necessary (e.g. if the `parent` was null checked already).
PR Close#39009
