fix(#56,#72): useSearchParams might return null and break script insertion

[feugy]

📓 What's in there?

Under some circumstances with pages router (some Next.js versions, env variable or depending on where the component is mounted), the following error is thrown when rendering SpeedInsights Next.js component:

TypeError: searchParams.entries is not a function
    at useRoute (file:///node_modules/.pnpm/@vercel+speed-insights@1.0.11_next@14.2.3_react@18.3.1/node_modules/@vercel/speed-insights/dist/next/index.mjs:151:40)
    at SpeedInsightsComponent (file:///node_modules/.pnpm/@vercel+speed-insights@1.0.11_next@14.2.3_react@18.3.1/node_modules/@vercel/speed-insights/dist/next/index.mjs:159:17)
    at renderWithHooks (/node_modules/.pnpm/react-dom@18.3.1_react@18.3.1/node_modules/react-dom/cjs/react-dom-server.browser.development.js:5658:16)
    at renderIndeterminateComponent (/node_modules/.pnpm/react-dom@18.3.1_react@18.3.1/node_modules/react-dom/cjs/react-dom-server.browser.development.js:5732:1

This PR fixes it, but providing a default empty search param object.

🧪 How to test?

Tested with:

• Next.js@14.2.3
• pages router
• component mounted in pages/_document.tsx

[vercel[bot]]

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Name	Status	Preview	Comments	Updated (UTC)
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[socket-security[bot]]

New and removed dependencies detected. Learn more about Socket for GitHub ↗︎

Package	New capabilities	Transitives	Size	Publisher
npm/@vercel/speed-insights@1.0.12	None	0	0 B

🚮 Removed packages: npm/@vercel/speed-insights@1.0.11

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