ci: Fix size limit action base upload

[mydea]

Noticed this is failing e.g. https://github.com/getsentry/sentry-javascript/actions/runs/8294578489/job/22700116991, it was an incorrect check: if there is no comparison branch, we don't need a PR.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback)	82.98 KB (added)
@sentry/browser (incl. Tracing, Replay)	78.5 KB (added)
@sentry/browser (incl. Tracing, Replay with Canvas)	81.82 KB (added)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	72.09 KB (added)
@sentry/browser (incl. Tracing)	78.5 KB (added)
@sentry/browser (incl. browserTracingIntegration)	78.5 KB (added)
@sentry/browser (incl. feedbackIntegration)	76.45 KB (added)
@sentry/browser (incl. feedbackModalIntegration)	76.64 KB (added)
@sentry/browser (incl. feedbackScreenshotIntegration)	76.65 KB (added)
@sentry/browser (incl. sendFeedback)	72.52 KB (added)
@sentry/browser	71.95 KB (added)
CDN Bundle (incl. Tracing, Replay, Feedback)	76.62 KB (added)
CDN Bundle (incl. Tracing, Replay)	71.48 KB (added)
CDN Bundle (incl. Tracing)	37.82 KB (added)
CDN Bundle	25.68 KB (added)
CDN Bundle (incl. Tracing, Replay) - uncompressed	224.12 KB (added)
CDN Bundle (incl. Tracing) - uncompressed	114.06 KB (added)
CDN Bundle - uncompressed	75.9 KB (added)
@sentry/react (incl. Tracing, Replay)	80.46 KB (added)
@sentry/react	73.93 KB (added)