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.

tests/test_tools_extended.py

@@ -265,6 +265,34 @@ async def test_extended_read_tools_failure_mode() -> None:
         await list_commits_tool(ErrorGitea(), {"owner": "acme", "repo": "app"})
 
 
+@pytest.mark.asyncio
+async def test_get_issue_tolerates_null_collections() -> None:
+    """Regression for #13: Gitea may return null for labels/assignees/user.
+
+    `.get(key, [])` returns None when the key is present with a null value, so
+    iterating the result raised `'NoneType' object is not iterable`.
+    """
+
+    class NullFieldsGitea(StubGitea):
+        async def get_issue(self, owner, repo, index):
+            return {
+                "number": index,
+                "title": "Issue",
+                "body": "Body",
+                "state": "open",
+                "user": None,
+                "labels": None,
+                "assignees": None,
+            }
+
+    result = await get_issue_tool(
+        NullFieldsGitea(), {"owner": "acme", "repo": "app", "issue_number": 1}
+    )
+    assert result["author"] == ""
+    assert result["labels"] == []
+    assert result["assignees"] == []
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize(
     "tool,args,expected_key",