Fix jumanpp.apply_to_sentence return empty list

tokenization_deberta_v2_jumanpp.py

@@ -27,4 +27,9 @@ class JumanppTokenizer:
         self.jumanpp = rhoknp.Jumanpp()
 
     def tokenize(self, text: str) -> str:
-        return " ".join([morpheme.surf for morpheme in self.jumanpp.apply_to_sentence(text).morphemes])
+        morphemes = self.jumanpp.apply_to_sentence(text).morphemes
+        if not morphemes:
+            doc = rhoknp.Document.from_raw_text(text)
+            morphemes = self.jumanpp.apply_to_document(doc).morphemes
+        return " ".join([morpheme.surf for morpheme in morphemes])
+