Nullable FQCNs throw illegal offset warnings

[hiddeco]

First of all, I introduced this bug myself in #122.

Nullable FQCNs currently throw lots of Illegal offset type in isset or empty warnings in the Astmap and ClassNameLayerResolverCacheDecorator because an object is passed as a class name which should be a string value.

The reason for this is that the NullableType::$type can either hold a string (as assumed in #122) or a FullyQualified node (as assumed before #122). The solution would be to check the type of NullableType::$type and take actions based on this type.

[timglabisch]

thanks =)