fix(useDropZone): improve argument type

[aki77]

<script setup lang="ts">
import { useDropZone } from '@vueuse/core'

const dropZoneRef = ref<HTMLElement>()

function onDrop(files: File[] | null) {
  // called when files are dropped on zone
}

const { isOverDropZone } = useDropZone(dropZoneRef, onDrop)
</script>

<template>
  <div ref="dropZoneRef">
    Drop files here
  </div>
</template>

This code is a type error.

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[aki77]

Thanks!