Fix bug in splitBlock
Fix bug in splitBlock

It would call `canSplit` with malformed arguments.

FIX: Fix crash in [`splitBlock`](##commands.splitBlock) when `defaultContentType` returns null.

See https://discuss.prosemirror.net/t/is-it-possible-to-alter-split-behavior-of-individual-nodes-marks/1165

ProseMirror/prosemirror-commands