Checklist

• I have searched the existing issues for similar issues.
• I added a very descriptive title to this issue.
• I have provided sufficient information below to help reproduce this issue.

Summary

Markdown rendering empty triple-backtick code fence adds the string "undefined".

Minimum example:

import streamlit as st

st.markdown("""
Hello

Has content:
```
some_content
```

Is empty, this one is the problem:
```
```

End
""")

Renders like so:

But would expect to have an empty code area, not a code area with the string "undefined" in it.

Reproducible Code Example

import streamlit as st

st.markdown("""
Hello

Has content:

some_content


Is empty, this one is the problem:



End
""")

Steps To Reproduce

1. Create the above streamlit app.py
2. Run it

Expected Behavior

Expect the empty triple-backtick code fence with nothing in it to show just an empty grey rectangle.

Current Behavior

Currently the empty triple-backtick code fence with nothing in it to shows a grey rectangle with the string "undefined".

Is this a regression?

• Yes, this used to work in a previous version.

Debug info

• Streamlit version: 1.51.0
• Python version: 3.10.19
• Operating System: Ubuntu 24.04.3 LTS
• Browser: Chrome

Additional Information

No response