ref(core): Make getTransport method on client optional

[kamilogorek]

I changed it to optional instead of just adding ?. check to make sure that we won't forget to do that in case we use it in other places. We do the same with sendSession already.

Closes #3995

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	22.32 KB (+0.01% 🔺)
@sentry/browser - Webpack	23.3 KB (0%)
@sentry/react - Webpack	23.33 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	29.77 KB (+0.02% 🔺)