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/src/exec/mutation.rs

@@ -26,6 +26,7 @@ fn literal_to_typed_array(
     num_rows: usize,
 ) -> Result<ArrayRef> {
     Ok(match (lit, data_type) {
+        (Literal::Null, _) => arrow_array::new_null_array(data_type, num_rows),
         (Literal::String(s), DataType::Utf8) => {
             Arc::new(StringArray::from(vec![s.as_str(); num_rows])) as ArrayRef
         }

crates/omnigraph/src/exec/projection.rs

@@ -74,6 +74,7 @@ fn evaluate_expr(batch: &RecordBatch, expr: &IRExpr, params: &ParamMap) -> Resul
 /// Create a constant array from a literal value.
 fn literal_to_array(lit: &Literal, num_rows: usize) -> Result<ArrayRef> {
     Ok(match lit {
+        Literal::Null => arrow_array::new_null_array(&DataType::Utf8, num_rows),
         Literal::String(s) => Arc::new(StringArray::from(vec![s.as_str(); num_rows])) as ArrayRef,
         Literal::Integer(n) => {
             // Try to match the most common integer types
@@ -283,6 +284,7 @@ fn list_scalar_type(items: &[Literal]) -> Result<ScalarType> {
 
 fn literal_scalar_type(lit: &Literal) -> Result<ScalarType> {
     match lit {
+        Literal::Null => Ok(ScalarType::String),
         Literal::String(_) => Ok(ScalarType::String),
         Literal::Integer(_) => Ok(ScalarType::I64),
         Literal::Float(_) => Ok(ScalarType::F64),

crates/omnigraph/src/exec/query.rs

@@ -1225,6 +1225,7 @@ fn ir_expr_to_sql(expr: &IRExpr, params: &ParamMap) -> Option<String> {
 
 pub(super) fn literal_to_sql(lit: &Literal) -> String {
     match lit {
+        Literal::Null => "NULL".to_string(),
         Literal::String(s) => format!("'{}'", s.replace('\'', "''")),
         Literal::Integer(n) => n.to_string(),
         Literal::Float(f) => f.to_string(),