ci: Skip gitflow steps if no PR was created

[mydea]

If you run the workflow manually, it will not create a PR if the branches are in sync, we should not error out in that case.

Also, we hard-code the source/target branches for the workflow instead of using "current branch" - as that would allow you to accidentally merge a different branch when triggering it as a workflow.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.87 KB (0%)
@sentry/browser - ES5 CDN Bundle (minified)	61.57 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.54 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	54.88 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.29 KB (0%)
@sentry/browser - Webpack (minified)	66.37 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.31 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.6 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.79 KB (0%)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.08 KB (0%)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	44.17 KB (0%)
@sentry/replay - Webpack (gzipped + minified)	38.92 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	61.52 KB (0%)