fix(compiler-core): fix crash when modifier is empty

[baiwusanyu-c]

close: #9865

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	87.1 kB	33.1 kB	29.9 kB
vue.global.prod.js	133 kB	50 kB	44.8 kB

Usages

Name	Size	Gzip	Brotli
createApp	48.4 kB	19 kB	17.4 kB
createSSRApp	51.6 kB	20.3 kB	18.5 kB
defineCustomElement	50.7 kB	19.8 kB	18.1 kB
overall	61.7 kB	23.9 kB	21.7 kB

[codspeed-hq]

CodSpeed Performance Report

Merging #9866 will not alter performance

_{Comparing baiwusanyu-c:bwsy/fix/emptyModifier (27b2503) with main (04d2c05)}

Summary

✅ 53 untouched benchmarks

[elizatlawy]

In which version this is deployed?

[EmilyRagan]

@elizatlawy looks like 3.3.13, according to the change log