Allow ByteString Variant to be None

[ldet]

Hello,
while working with an open62541 based server, I encountered an empty/null ByteString. The ua_binary implementation of ByteString (_Bytes) already handles this case as per the standard (Part 6 5.2.2.7) and sets the value to None, but the implementation of Variant currently throws an UaError when a ByteString with value None is encountered. This PR adds ByteString to the allowed VariantTypes that can be None.