The ModuleSerializer does not support serializing anonymous classes
because when we try to deserialize the anonymous class, it wouldn't
know which class to use (since class name is nil).

For this reason, ModuleSerialzier now raises an error if the class
name is nil. Previously, ModuleSerializer would raise an `undefined
method `constantize' for nil:NilClass` error during deserialization.
It's not clear why the deserialization failed from the error.

In this commit, we raise an explicit error when trying to serialize
an anonymous class indicating this behaviour is not supported.