[desk-tool] Fix issue where non-function child would fail to resolve

[rexxars]

The following structure fails to resolve the list:

import S from '@sanity/desk-tool/structure-builder'

export default S.list()
  .title('Content')
  .items([
    S.listItem()
      .title('Posts')
      .schemaType('post')
      .child(
        S.documentTypeList('category')
          .title('Posts by category')
          .child(S.list().title('test'))
      ),

    ...S.documentTypeListItems()
  ])

The following works:

import S from '@sanity/desk-tool/structure-builder'

export default S.list()
  .title('Content')
  .items([
    S.listItem()
      .title('Posts')
      .schemaType('post')
      .child(
        S.documentTypeList('category')
          .title('Posts by category')
          .child(() => S.list().title('test'))
      ),

    ...S.documentTypeListItems()
  ])

This was caused by us explicitly checking whether the child was a function, which it shouldn't have to be. Changed to only care about it being truthy or not.