fix(amazonq): accept empty body partial result. (#7158)

## Problem
If the server sends a partial result without a body, we currently ignore
it.

## Solution
- explicitly check for undefined. 

---

- Treat all work as PUBLIC. Private `feature/x` branches will not be
squash-merged at release time.
- Your code changes must meet the guidelines in
[CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).
- License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: Hweinstock

packages/amazonq/src/lsp/chat/messages.ts

@@ -516,7 +516,7 @@ async function handlePartialResult<T extends ChatResult>(
             ? await decodeRequest<T>(partialResult, encryptionKey)
             : (partialResult as T)
 
-    if (decryptedMessage.body) {
+    if (decryptedMessage.body !== undefined) {
         void provider.webview?.postMessage({
             command: chatRequestType.method,
             params: decryptedMessage,