[web-animations-1] null or undefined keyframe in iterable keyframe object causes an assertion failure #14113

Steps 5.3.6 and 5.3.7 of process a keyframes argument state:

6. If Type(nextItem) is not Undefined, Null or Object, then throw a TypeError and abort these steps.
7. Append to processed keyframes the result of running the procedure to process a keyframe-like object passing nextItem as the keyframe input with the allow lists flag set to false.

But process a keyframe-like object explicitly only takes an object, not null or undefined. And indeed it eventually calls EnumerableOwnNames on its input argument (Step 3), which fails an assertion when the input is not an object.