fix: add optional chain to prevent nullish broken

[ShenQingchuan]

I have encountered this issue in my own PPT project (which can be a reproduction repo): https://github.com/ShenQingchuan/vue-vine-ppt-2025

But it'll be OK after restarting the slidev dev server.

I think it might be caused by my Vue Vine plugin using in setup/main.ts, maybe it's applied before Vue Router and the router instance was not initialized at that moment? I'm not quite sure ...

But I've tried and approved that this line of change works as expected, which is adding optional chain operators.

fix #2047

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[kermanx]

Seems to be #2047

[kermanx]

Seems to be #2047

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commit: c2194a9