gen_release_notes: avoid crashing when none of the commits mention closing an issue

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20459>

bin/gen_release_notes.py

@@ -217,7 +217,10 @@ async def parse_issues(commits: str) -> typing.List[str]:
 
 async def gather_bugs(version: str) -> typing.List[str]:
     commits = await gather_commits(version)
-    issues = await parse_issues(commits)
+    if commits:
+        issues = await parse_issues(commits)
+    else:
+        issues = []
 
     loop = asyncio.get_event_loop()
     async with aiohttp.ClientSession(loop=loop) as session: