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

Author: ChristianKohler

src/internal/Subscription.ts

@@ -130,6 +130,11 @@ export class Subscription implements SubscriptionLike {
  */
 add(teardown: TeardownLogic): Subscription{
 letsubscription=(<Subscription>teardown);
+
+if(!(<any>teardown)){
+returnSubscription.EMPTY;
+}
+
 switch(typeofteardown){
 case'function':
 subscription=newSubscription(<(()=>void)>teardown);
@@ -147,9 +152,6 @@ export class Subscription implements SubscriptionLike {
 }
 break;
 default: {
-if(!(<any>teardown)){
-returnSubscription.EMPTY;
-}
 thrownewError('unrecognized teardown '+teardown+' added to Subscription.');
 }
 }