Check whether node.children is undefined

[alexander-turner]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)
• If applicable, I’ve added docs and tests (Note: I don't see where to add a test to make sure this gets caught)

Description of changes

A function wasn't checking that a property was defined before trying to access that property's length field. The function now returns the empty list if the property is undefined.

[wooorm]

Hey Alex! The function is only called on nodes with children. You are apparently seeing some issue. There is a root problem somewhere. You can try patching it here, but you’ll run into it everywhere. Better to figure out the root cause. Can you do some debugging and figure out why you have parent nodes without children?

[wooorm]

Closing as this is probably a problem in your code somewhere else!