[ZEPPELIN-6404] Fix empty response body handling in merge_pr.py

JIRA transition API returns empty response body on success, which
caused a JSONDecodeError. Return empty dict when body is empty.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jongyoul

dev/merge_pr.py

@@ -92,11 +92,12 @@ def _http(self, method, url, payload=None, auth=""):
             req.add_header("Authorization", auth)
         try:
             with urllib.request.urlopen(req) as resp:
-                return resp.status, json.loads(resp.read().decode())
+                body = resp.read().decode()
+                return resp.status, json.loads(body) if body else {}
         except urllib.error.HTTPError as e:
             err_body = e.read().decode() if e.fp else ""
             try:
-                return e.code, json.loads(err_body)
+                return e.code, json.loads(err_body) if err_body else {}
             except json.JSONDecodeError:
                 return e.code, {"error": err_body}