fix empty lesson info paragraphs/messages

backend/lesson_info.py

@@ -585,7 +585,7 @@ def from_str(cls, paragraph: str, lesson: models.Lesson, index: int,
                  plan_type: typing.Literal["forms", "teachers", "rooms"]) -> LessonInfoParagraph:
         messages = [
             LessonInfoMessage.from_str(message.strip(), lesson, i, plan_type)
-            for i, message in enumerate(paragraph.split(","))
+            for i, message in enumerate(paragraph.split(",")) if message.strip()
         ]
         new_messages = []
         for message in messages:
@@ -620,7 +620,7 @@ def from_str(cls, info: str, lesson: models.Lesson, plan_type: typing.Literal["f
                  ) -> ParsedLessonInfo:
         return cls([
             LessonInfoParagraph.from_str(paragraph.strip(), lesson, i, plan_type)
-            for i, paragraph in enumerate(info.split(";"))
+            for i, paragraph in enumerate(info.split(";")) if paragraph.strip()
         ])
 
     def serialize(self, lesson_date: datetime.date) -> list: