This usually means that an `ERR_FILE*` error occurred.
Previously, using `GDScriptCache::get_full_script()` would ignore errors during loading.
Now, all errors are not ignored.
Judging in which period the error occurred, it can be judged based on the return value:
1. null + err : Error during script loading (load_source_code()).
2. script + err: Error during script parsing.
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