fix: Defer tracing decision to downstream SDKs when using SDK without performance

[lforst]

Currently, since the introduction of "Tracing without Performance", we always set a negative sampling decision on the tracing context when there is not an active transaction. This could get problematic because we might overrule the sampling decisions for all downstream SDKs when there are no active transactions and when we're the head-SDK.

Per spec, it is allowed to leave away the sampling decision in the sentry-trace header. The use-case for this is to defer the sampling decision, which is exactly what we want to do.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	75.18 KB (+0.44% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.16 KB (+0.16% 🔺)
@sentry/browser - Webpack (gzipped)	21.84 KB (+0.16% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	69.72 KB (+0.26% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	28.17 KB (+0.08% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	20.18 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	219.93 KB (+0.4% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	84.75 KB (+0.11% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	59.85 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.04 KB (+0.07% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	75.2 KB (+15.6% 🔺)
@sentry/react - Webpack (gzipped)	21.87 KB (+0.15% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	93.02 KB (+0.4% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	50.69 KB (+0.17% 🔺)