Fix ValueError exception on empty Korean text. (#4245)

spacy/lang/ko/__init__.py

@@ -58,7 +58,8 @@ def check_spaces(text, tokens):
             yield prev_end != idx
         prev_end = idx + len(token)
         start = prev_end
-    yield False
+    if start > 0:
+        yield False
 
 
 class KoreanTokenizer(DummyTokenizer):

spacy/tests/lang/ko/test_tokenizer.py

@@ -45,3 +45,8 @@ def test_ko_tokenizer_full_tags(ko_tokenizer, text, expected_tags):
 def test_ko_tokenizer_pos(ko_tokenizer, text, expected_pos):
     pos = [token.pos_ for token in ko_tokenizer(text)]
     assert pos == expected_pos.split()
+
+
+def test_ko_empty_doc(ko_tokenizer):
+    tokens = ko_tokenizer("")
+    assert len(tokens) == 0