Fix resource references not being loaded on exported builds (Godot 4.3)

[shomykohai]

Description

On exported builds, it is not guaranteed that the get_default_value returns a Resource, even if we cast it.
In fact, in the editor they are treated as Resources, but on exported builds they are Strings.
This prevents casting a String as a Resource by calling load() if default_value is not a Resource.

Edit: The issue seem to be related to the same one as #187, the generated GDScript bytecode on exported builds.

Screenshots

Printing out default_value before this PR:
EDITOR:

EXPORTED BUILD:

[shomykohai]

Superseded by #197.