fix(dismissable-layer): guard isTopMostLayer against empty layers array

[EduardF1]

Problem

isTopMostLayer crashes with TypeError when layers array is empty.

Root cause

layers[layers.length - 1].node throws when layers is empty, since layers[-1] is undefined. Can happen during teardown or before a layer is registered.

Fix

Add a length guard:

function isTopMostLayer(node: HTMLElement | null) {
  return layers.length > 0 && layers[layers.length - 1].node === node;
}
```n
Returning `false` for an empty stack is correct — no layer is on top.

Fixes #649

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🔨 Latest commit	7eadb77

[EduardF1]

Hi maintainers, friendly nudge on this small fix. The change guards isTopMostLayer against the empty-layers array case that surfaces in the linked issue, with no behavioral change when there are layers. Happy to iterate if any naming or test placement should be adjusted.

[EduardF1]

Friendly ping on this one. The fix is a small defensive guard:
isTopMostLayer was reading layers[layers.length - 1] without checking
for an empty array, which I hit after programmatically closing the
last dismissable layer. One-line change, no behavior shift in the
happy path. Happy to add a test or rebase if useful. Thanks for the
work on this lib!