Fix mypy errors in the tests

The config values are now a TypedDict, so check inputs for None before sorting, use literal keys and use ConfigArgs as the fake _read_config() return type.

tests/test_cli.py

@@ -91,7 +91,7 @@ def config_calls(monkeypatch: pytest.MonkeyPatch) -> list[tuple[str, Optional[st
     calls: list[tuple[str, Optional[str]]] = []
     read_config = sb._read_config
 
-    def record(config_file: str, user: Optional[str] = None) -> dict[str, Any]:
+    def record(config_file: str, user: Optional[str] = None) -> sb.ConfigArgs:
         calls.append((config_file, user))
 
         return read_config('/nonexistent/simple_backup.conf', user)