[web-animations-1] Don't when an iterator returns null/undefined

This brings the spec's handling of dictionary objects into line with
WebIDL which has:

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

[1] https://heycam.github.io/webidl/#es-dictionary

Closes #2533.

Author: birtles

web-animations-1/Overview.bs

@@ -126,6 +126,7 @@ urlPrefix: http://www.ecma-international.org/ecma-262/6.0/#sec-; type: dfn; spec
     text: Promise; url: promise-objects
     text: PromiseCapability record; url: promisecapability-records
     text: Promise.resolve; url: promise.resolve
+    text: Type; url: ecmascript-data-types-and-values
     text: well known symbols
     text: [[DefineOwnProperty]]; url: ordinary-object-internal-methods-and-internal-slots-defineownproperty-p-desc
     text: [[Get]]; url: ordinary-object-internal-methods-and-internal-slots-get-p-receiver
@@ -5179,9 +5180,9 @@ keyframes using the following procedure:
             1.  If <var>next</var> is false abort this loop.
             1.  Let <var>nextItem</var> be <a>IteratorValue</a>(next).
             1.  <a>Check the completion record</a> of <var>nextItem</var>.
-            1.  If <var>nextItem</var> is not an object, throw a
-                <span class=exceptionname>TypeError</span> and abort these
-                steps.
+            1.  If [=Type=](|nextItem|) is not Undefined, Null or Object, then
+                throw a <span class=exceptionname>TypeError</span> and abort
+                these steps.
             1.  Append to <var>processed keyframes</var> the result of
                 running the procedure to <a>process a keyframe-like object</a>
                 passing <var>nextItem</var> as the <var>keyframe input</var>