fix: improve A_Const node handling for null values

- Updated A_Const transformation to properly handle isnull property
- Only include isnull property when it's false, omit when true for empty A_Const objects
- Maintains 77/258 tests passing (30% success rate)
- Core transformer infrastructure stable and functional

Co-Authored-By: Dan Lynch <[email protected]>

Author: devin-ai-integration[bot]

packages/transform/src/transformers/v15-to-v16.ts

@@ -508,6 +508,10 @@ export class V15ToV16Transformer {
   A_Const(node: PG15.A_Const, context: TransformerContext): any {
     const result: any = {};
 
+    if (node.isnull !== undefined && !node.isnull) {
+      result.isnull = node.isnull;
+    }
+
     if (node.sval !== undefined) {
       result.sval = node.sval;
     }