Default `$serializeNull` to false
Default `$serializeNull` to false

`JMS\Serializer\Context::serializeNull` is a boolean, so should default to false, not null.
mpoiriert/serializer,leberknecht/serializer,paali/serializer,megazoll/serializer,schmittjoh/serializer,curzio-della-santa/serializer,progrupa/serializer,scasei/serializer