fix(nextjs): Check for validity of API route handler signature

[lforst]

We had reports of res being undefined inside the withSentry wrapper so just to be sure we should check for the existence of req and res.
These objects might be undefined if users manually call the API handlers without passing along the req and res objects.

[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 (+0.01% 🔺)
@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 (-0.01% 🔽)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	92.93 KB (+0.31% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	50.66 KB (+0.1% 🔺)