fix: tolerate null labels/assignees/user in get_issue (#13)

Gitea may return JSON null for an issue's `labels`, `assignees`, or
`user` fields. `dict.get(key, [])` returns None when the key is present
with a null value (the default is only used for missing keys), so the
list comprehensions raised `'NoneType' object is not iterable` for
otherwise-valid issues. Coalesce with `or []` / `or {}` so empty/null
collections normalize to empty results.

Adds a regression test covering all three null fields.

src/aegis_gitea_mcp/tools/read_tools.py

@@ -272,9 +272,9 @@ async def get_issue_tool(gitea: GiteaClient, arguments: dict[str, Any]) -> dict[
             "title": limit_text(str(issue.get("title", ""))),
             "body": limit_text(str(issue.get("body", ""))),
             "state": issue.get("state", ""),
-            "author": issue.get("user", {}).get("login", ""),
-            "labels": [label.get("name", "") for label in issue.get("labels", [])],
-            "assignees": [assignee.get("login", "") for assignee in issue.get("assignees", [])],
+            "author": (issue.get("user") or {}).get("login", ""),
+            "labels": [label.get("name", "") for label in (issue.get("labels") or [])],
+            "assignees": [assignee.get("login", "") for assignee in (issue.get("assignees") or [])],
             "created_at": issue.get("created_at", ""),
             "updated_at": issue.get("updated_at", ""),
             "url": issue.get("html_url", ""),