[web-animations-1] Don't throw for custom iterator that return null/undefined

[stephenmcgruer]

(From discussion on web-platform-tests/wpt#10399)

Currently the procedure to process a keyframes argument (https://drafts.csswg.org/web-animations-1/#processing-a-keyframes-argument) says:

(in the otherwise case)
5.4.6 "If nextItem is not an object, throw a TypeError and abort these steps."

However in the above linked PR, Brian noted:

"Yeah, it looks like we shouldn't throw on null / undefined.

In WebIDL when we create a sequence from an iterable we have the step:

4. Initialize Si to the result of converting nextItem to an IDL value of type T.

For converting dictionary types we have:

1. If Type(V) is not Undefined, Null or Object, then throw a TypeError.

So we should update the Web Animations spec to match this and add a test for this case that we don't throw."