Fix #safe_constantize failing for Object scoped missing constants

- Before
 "Object::ABC".safe_constantize # => NameError: uninitialized constant ABC
The output should be nil.
- After
"Object::ABC".safe_constantize # => nil

This issue only happens if `Object` is very first item:

`"A::Object::B".safe_constantize => nil`

So simpler to do `parts.shift if parts[0] == "Object"`