pNext: "Member isn't allowed to be filled!" incorrect behavior when 'null'

[maierfelix]

This error should only popup, when a user actually tries to fill the pNext member with another structure. Currently, when a non-writeable pNext member gets set to null, this error shows

Example:

let accelerationStructureInfo = new VkAccelerationStructureCreateInfoNV();
accelerationStructureInfo.pNext = null; // shouldn't trigger an error

Proposal:
Only trigger the error, when the passed in value is not equal to null