Strings (essentially pointers) as arguments can be nullable

[RamyElkest]

For instance virDomainGetMetadata takes a uri string which is conditionally nullable

This should apply to virConnectOpen as well

This will probably apply to all pointer types

This means in our type check we should check for napi_null/napi_undefined as well as the type

[RamyElkest]

Note: additionally interface/wrapper arguments should default to null