[ISSUE] SearchWarning: Cannot read properties of undefined (reading 'get') in line #69

### Steps to reproduce

From [`src/components/views/elements/SearchWarning.tsx#L69`](https://github.com/matrix-org/matrix-react-sdk/blob/ad190b1dcc8aa332b9492c9dc8508e25c2914d94/src/components/views/elements/SearchWarning.tsx#L69), the exception `TypeError: Cannot read properties of undefined (reading 'get')` is thrown when clicking on the room header search button (`mx_RoomHeader_searchButton`) if `desktop_builds` is not defined in config.

Adding the optional chaining operator `?.` in line # 69, like `if (desktopBuilds?.get("available")) {`, would fix this issue.

### Outcome

Exception

### Operating system

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### Browser information

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### URL for webapp

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### Application version

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### Homeserver

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### Will you send logs?

No