fix: handle undefined hash in socket message handler

[MIGUELez11]

What

Fixed handling of undefined hash values received from rspack during hot module reload.

Why

When rspack sends an undefined hash through WebSocket messages, it causes a TypeError when trying to use indexOf on the undefined value. This happens in the following sequence:

1. Run the dev-server

2. Visit the page

3. Rspack triggers a HMR and for some reason returns an empty hash

4. The hash is handled and when triggering a reloadApp the currentHash is undefined

How

Added propert handling for undefined hash values in the hot update handler to prevent the TypeError.

Notes

I've used --no-verify for the commit as the file contains pre-existing style inconsistencies that are outside the scope of this fix.

Screenshots

The messages sent from Rspack via the WebSocket showing a type hash without data

The error thrown and catched with react-refresh of reloadApp trying to call currentHash.indexOf(/** @type {string} */ previousHash) >= 0

[CLAassistant]

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[LingyuCoder]

Thanks for your contribution