It's perfectly valid for an abstract control not to have a defined parent; yet previously the
types were asserting that AbstractControl#parent is not a null value. This changes correctly
reflects the run-time behavior through the types.
BREAKING CHANGE: Type of AbstractFormControl.parent now includes null
`null` is now included in the types of .parent. If you don't already have a check for this case,
the TypeScript compiler might compain. A v11 migration exists which adds the not-null assertion
operator where necessary.
In an unlikely case your code was testing the parnet against undefined with sitrct equality,
you'll need to change this to `=== null` instead, since the parent is not explicily initialized
with `null` instead of being left `undefined`.
Fixes#16999
PR Close#32671
f4f1bcc997