bug: modifying empty doc fails

[YousefED]

See testcase test.fails("suggestion mode: add heading to empty doc",

https://github.com/TypeCellOS/BlockNote/blob/13e35998dd9ff44272815a0947f271411f75100e/tests/src/browser/y-prosemirror/addRemoveBlocks.test.tsx#L24

[YousefED]

currently fails with TransformError: No node at mark step's position