[scroll-animations] allow explicitly setting null as a source when constructing a ScrollTimeline (#11357)

It should be possible to explicitly set a `null` source when constructing a `ScrollTimeline`. This addresses #11340.

Author: graouts

scroll-animations-1/Overview.bs

@@ -311,7 +311,7 @@ spec:selectors-4; type:dfn; text:selector
 			1.  Set the {{ScrollTimeline/source}} of |timeline| to:
 
 				<dl class="switch">
-					:   If the `source` member of |options| is present and not null,
+					:   If the `source` member of |options| is present,
 					::  The `source` member of |options|.
 
 					:   Otherwise,