rootKey in getTreeFromFlatData not behaving correctly for string based keys

Hi, think I've found a bug,

When trying to use the getTreeFromFlat data function, and:
1. Your id's are global strings
2. Your tree doesn't have a rootKey

The function keeps returning back an empty array []

I think this is a bug in the function in the following code block
https://github.com/fritz-c/react-sortable-tree/blob/337a633e75a761f6a10bdf9ea0e2d06d82e603b9/src/utils/tree-data-utils.js#L993

I think this shouldn't return an empty array if no rootKey is give.

I've also tried setting it to '0' as per #189 but that's not helping either.

Any ideas what else it could be?

Thanks