fix(wren-core-wasm): preserve SQL NULLs in JSON output and update release docs

- Change with_explicit_nulls(false) to true so nullable columns
  serialize as {"amount": null} instead of being silently dropped
- Add wren-core-wasm to RELEASING.md publish workflow list

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: goldmedal

RELEASING.md

@@ -50,6 +50,7 @@ Merging a Release PR triggers:
   - ibis-server → multi-arch Docker image pushed to GHCR (tagged `<version>` + `latest`)
   - wren-core-py → multi-platform wheels + sdist published to PyPI
   - wren → sdist + wheel published to PyPI
+  - wren-core-wasm → WASM package published to npm
   - mcp-server → version verification
 
 ## RC (pre-release) workflow

wren-core-wasm/src/lib.rs

@@ -452,7 +452,7 @@ impl WrenEngine {
 
         let mut buf = Vec::new();
         let mut writer = WriterBuilder::new()
-            .with_explicit_nulls(false)
+            .with_explicit_nulls(true)
             .build::<_, JsonArray>(&mut buf);
 
         for batch in &batches {