fix(nextjs): Only re-export default export if it exists

[lforst]

Injecting the Sentry config code failed when there wasn't a default export in a file that is supposed to be wrapped (e.g. a middleware file that only has a middleware export).

This PR fixes this by accessing the .default property dynamically and either re-exporting the default export or undefined.

[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)	69.72 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% 🔺)