fix(utils): Make empty traceparent string count as invalid

[lforst]

So far, an empty sentry-trace header counted as valid in the SDK, in a sense that the extractTraceparentData function still returned an object instead of undefined.

This PR changes the behavior so that an empty string is not counted as valid.

Unblocks #5710

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.53 KB (+0.01% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	60.3 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.12 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	53.22 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	19.91 KB (0%)
@sentry/browser - Webpack (minified)	64.61 KB (0%)
@sentry/react - Webpack (gzipped + minified)	19.93 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	44.98 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.1 KB (+0.02% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.48 KB (+0.02% 🔺)

[lforst]

This was too messy to do separately