code: def test_issue309(en_vocab):
      """Test Issue #309: SBD fails on empty string"""
      doc = Doc(en_vocab, words=[" "], heads=[0], deps=["ROOT"])
      assert len(doc) == 1
      sents = list(doc.sents)
      assert len(sents) == 1
  
imports: import pytest
  from spacy.tokens import Doc
  from ...util import apply_transition_sequence
label_desc: Use image node_id 1 for calling a global function with example usage: test_issue309(en_vocab) without return types