Fix nullable query parameters: accept omission and null for ? params

Parameters declared with `?` (e.g. `$changelogUrl: String?`) now correctly
accept omission or explicit null in JSON input instead of requiring empty
strings as a workaround. Adds `Literal::Null` variant and threads it through
parameter parsing, type-checking, and Arrow array conversion.

https://claude.ai/code/session_014oGFKL7EVg1b2cyPgt9Gne

crates/omnigraph-compiler/src/query/ast.rs

@@ -160,6 +160,7 @@ impl std::fmt::Display for AggFunc {
 
 #[derive(Debug, Clone)]
 pub enum Literal {
+    Null,
     String(String),
     Integer(i64),
     Float(f64),