Fix: "start from scratch" button results in a "no query provided" error

[neubig]

Description

This PR fixes issue #7621 where clicking the "start from scratch" button on the hero page results in a "no query provided" toast error.

Changes

• Added an allowEmptyQuery parameter to the useCreateConversation hook to allow empty queries when explicitly starting from scratch
• Updated the CodeNotInGitHubLink component to use the new parameter
• Added tests to verify the fix works correctly

Fixes #7621

@neubig can click here to continue refining the PR

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:d6935e3-nikolaik   --name openhands-app-d6935e3   docker.all-hands.dev/all-hands-ai/openhands:d6935e3

[neubig]

I have not been able to reproduce this error recently, so I assume it's fixed now