fix(tracing): Account for case where startTransaction returns undefined

[AbhiPrasad]

if you have not enabled tracing extensions via addTracingExtensions, this function will not generate spans, and the span returned from the callback may be undefined.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	20.44 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	63.52 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.13 KB (-0.01% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	56.57 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	21.59 KB (0%)
@sentry/browser - Webpack (minified)	71.65 KB (+0.01% 🔺)
@sentry/react - Webpack (gzipped + minified)	21.61 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	51.85 KB (-0.21% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	27.77 KB (+0.05% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.08 KB (+0.06% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	44.21 KB (0%)
@sentry/replay - Webpack (gzipped + minified)	38.28 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	62.39 KB (+0.02% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	55.95 KB (0%)