Creating a new document is not possible for a documentList with children

[samimakela]

Describe the bug

When clicking "Create new document" in a documentList pane with a child, the child pane will open instead of the document editor.

This is problematic because a lot of Sanity documentation instructs to build filtered lists like this. E.g. https://www.sanity.io/docs/structure-builder-cheat-sheet#68ee42d8c7a5

To Reproduce

Steps to reproduce the behavior in test-studio.

1. Click 'Custom panes'
2. Click 'Books by author'
3. Click '+' icon next to author
4. Click 'Author'

Expected behavior

Expected the document editor to be opened. Please compare to behaviour with the following steps in test-studio.

1. Click 'Custom panes'
2. Click 'Authors & Books'
3. Click '+' icon next to 'Authors & Books'
4. Click 'Author'

Which versions of Sanity are you using?

Run sanity versions in the terminal and copy-paste the result here.

@sanity/cli (global) 3.78.0 (latest: 3.78.1)
@sanity/eslint-config-i18n 1.0.0 (latest: 1.1.0)
@sanity/eslint-config-studio 4.0.0 (latest: 5.0.2)
@sanity/mutate 0.12.1 (up to date)
@sanity/pkg-utils 6.13.4 (latest: 7.0.4)
@sanity/prettier-config 1.0.3 (up to date)
@sanity/test 0.0.1-alpha.1 (up to date)
@sanity/tsdoc 1.0.169 (latest: 2.0.9)
@sanity/ui 2.15.2 (up to date)
@sanity/uuid 3.0.2 (up to date)
sanity 3.78.1 (up to date)

Additional context

getIntentState() in packages/sanity/src/structure/getIntentState.ts seems to expect that all documentLists with matching type and filter are able to handle the create intent. This is not the case if the documentList has children as the steps to reproduce mentioned above shows.

Adding && pane.child === undefined to the statement seems to fix the issue.