ref(node-experimental): Remove custom isInitialized

[mydea]

We have this in core now, and align the implementation there to work consistently (and also in a future where a client may not be undefined).

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.95 KB (+0.14% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	69.18 KB (+0.15% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	73.11 KB (+0.15% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.79 KB (+0.16% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	33.37 KB (+0.3% 🔺)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	33.23 KB (+0.31% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.16 KB (+0.07% 🔺)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.16 KB (+0.08% 🔺)
@sentry/browser - Webpack (gzipped)	22.43 KB (+0.1% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	76.09 KB (+0.38% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	67.6 KB (+0.35% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	33.59 KB (+0.77% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	24.64 KB (+0.86% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	212.93 KB (+0.28% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	101.55 KB (+0.59% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	73.83 KB (+0.61% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	36.83 KB (+0.75% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	69.55 KB (+0.16% 🔺)
@sentry/react - Webpack (gzipped)	22.46 KB (+0.09% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	87.34 KB (+0.1% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	50.45 KB (+0.14% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	17.1 KB (+0.01% 🔺)