fix: filter empty string when generating delimiter for sub

Author: icelam

app/utils/markdown.py

@@ -40,7 +40,7 @@ def convert_pre(self, el, text, convert_as_inline):
         return ('\n' if not after_paragraph else '') + f"```{self.options['code_language']}\n{unescaped_text}\n```\n\n"
 
     def convert_sub(self, el, text, convert_as_inline):
-        return f'_{text}'
+        return f'_{text}' if text.strip() else text
 
     def convert_sup(self, el, text, convert_as_inline):
         return f'^{text}'