fix: return null when parameter is unbound

core/error.rs

@@ -1,5 +1,3 @@
-use std::num::NonZero;
-
 use thiserror::Error;
 
 #[derive(Debug, Error, miette::Diagnostic)]
@@ -49,8 +47,6 @@ pub enum LimboError {
     Constraint(String),
     #[error("Extension error: {0}")]
     ExtensionError(String),
-    #[error("Unbound parameter at index {0}")]
-    Unbound(NonZero<usize>),
     #[error("Runtime error: integer overflow")]
     IntegerOverflow,
     #[error("Schema is locked for write")]