Empty command process

Before the stringify overloading, it was possible to make an process instance with empty command list and this would be boolean equivalent of true.

With new stringify and autogeneration of bool overload using the stringify by perl, this is no longer possible: `_initialize` returned `$self`, which evaluated to `false` in the `CAF::Object::new` and throw a stack trace.

This PR allows to initialize a process instance with empty command, but this instance still is still `false`.