fix(Subscription): Return Empty when teardown === null (#4575)
fix(Subscription): Return Empty when teardown === null (#4575)

* fix(Subscription): Return Empty when teardown === null

`typeof teardown` returns object when null. It breaks later when rxjs tries to access the closed property.

The commit fixes that by first checking for null/undefined.

* fix(Subscription): tslint errors

kwonoj/RxJS,martinsik/rxjs,benlesh/RxJS,ReactiveX/RxJS,trxcllnt/RxJS,ReactiveX/rxjs,ReactiveX/rxjs,kwonoj/RxJS,ReactiveX/rxjs,ReactiveX/rxjs,benlesh/RxJS,martinsik/rxjs,kwonoj/RxJS,ReactiveX/RxJS,ReactiveX/RxJS,martinsik/rxjs,benlesh/RxJS,trxcllnt/RxJS,trxcllnt/RxJS,martinsik/rxjs,ReactiveX/RxJS,trxcllnt/RxJS,kwonoj/RxJS,benlesh/RxJS