fix(core): Add guard against scope.getAttachments

[AbhiPrasad]

Fixes #5229 (comment)

We need to check for the existence of finalScope.getAttachments because getAttachments can be undefined if users are using an older version of @sentry/core that does not have the getAttachments method.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.52 KB (+0.02% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	60.39 KB (+0.03% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.18 KB (+0.04% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	53.74 KB (+0.04% 🔺)
@sentry/browser - Webpack (gzipped + minified)	19.91 KB (+0.02% 🔺)
@sentry/browser - Webpack (minified)	65.2 KB (+0.03% 🔺)
@sentry/react - Webpack (gzipped + minified)	19.94 KB (+0.01% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	45.93 KB (+0.01% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.36 KB (+0.02% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.77 KB (+0.03% 🔺)