fix(nextjs): Check for existence of default export when wrapping pages

[lforst]

Fixes #8792

In #8792 a user reports that we read getInitialProps from undefined. The only place where we read getInitialProps is in the page wrapper template. As a fix we simply check for its existence before accessing the field. We don't fully know yet how this can happen, but I guess it can't hurt to introduce this check.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	75.12 KB (+0.36% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.14 KB (+0.09% 🔺)
@sentry/browser - Webpack (gzipped)	21.81 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	69.73 KB (+0.27% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	28.18 KB (+0.1% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	20.18 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	219.89 KB (+0.38% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	84.78 KB (+0.13% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	59.86 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.04 KB (+0.09% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	65.3 KB (+0.39% 🔺)
@sentry/react - Webpack (gzipped)	21.84 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	92.94 KB (+0.32% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	50.66 KB (+0.1% 🔺)