Enhance output handling: update tool result check to handle null and undefined cases

Author: habedi

src/agent/state.ts

@@ -265,7 +265,7 @@ export const useStore = create<AppState & AppActions>((set, get) => ({
                                 config,
                             );
                             const output =
-                                result === ""
+                                result === "" || result === null || result === undefined
                                     ? "Tool executed successfully and produced no output."
                                     : result;
                             toolResults.push({