Editorial: Remove unnecessary undefined type from property descriptor tests

[anba]

Is{Accessor,Data,Generic}Descriptor are never called with undefined.

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StructuredSerializeInternal seems to be the only caller where undefined is ever used as an input.

[bakkot]

IsDataDescriptor is called in step 2.b of [[DefineOwnProperty]] for Arrays and step 8 of ArraySetLength passing a value which the reader does not know not to be undefined. I would change both of those steps to something like Assert: _lengthDesc_ is not *undefined* and IsDataDescriptor(_lengthDesc_) is *true*.