Fix empty/only-comments AST generation

[mla2001]

Initial Checks

• I have searched existing issues for duplicates
• I have read the documentation

Summary

Fix generic error message in ASTConstructor when AST construction fails with an empty script or only with comments.

Reproducible Example

script = "//Comment"
create_ast(script)

Expected vs Actual

Expected: Successful empty execution
Actual: Uncontrolled exception

vtlengine version

1.6.0

Python version

Any

OS

Any