fix(nextjs): Guard for case where getInitialProps may return undefined

[lforst]

Fixes #9066

Consideration: If users return undefined we will override that with an object, which is arguably fine since when undefined is returned you will likely not depend on it in any way.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	82.66 KB (+0.18% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	71.77 KB (+0.16% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	30.94 KB (+0.96% 🔺)
@sentry/browser - Webpack (gzipped)	21.26 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	73.03 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	28.93 KB (+0.78% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	21.09 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	233.81 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	87.77 KB (+0.77% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	62.76 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.71 KB (+0.81% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	83.05 KB (+0.09% 🔺)
@sentry/react - Webpack (gzipped)	21.29 KB (+0.03% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	99.43 KB (+0.05% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	47.83 KB (+0.58% 🔺)

[Lms24]

Sounds reasonable!