Add nullable information to ArgumentMetadata

[fritz-gerneth]

Working on some custom ArgumentResolver, it would be good to know if an argument is nullable. This would allow us to either return null or throw an exception in the ArgumentResolver::resolve call (e.g. when the header is not present in the message). Right now this does throw some Type error that null is given.

Proposed Change: extend ArgumentMetadata with an additional property bool $allowsNull (same name as the method in the ReflectionParameter)

[DanielBadura]

Yeah, this sounds good 👍

[DanielBadura]

closed by #812