This IDL change is needed for webaudio's WaveShaperNode, where a
null BufferSource for a curve attribute results in a zero-length buffer.
WebGL also has a nullable BufferSource arg in bufferData(...). But
there, a null data/srcData value returns GL_INVALID_VALUE.
By making use of the WEB_PLATFORM_OBJECT macro we can remove
the boilerplate of needing to add this override for every
serializable platform object so that we can check whether they
are exposed or not.
