chore: do not send message for xlarge PR

[waynexia]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Set the message on xlarge PR to empty string, discussion: CodelyTV/pr-size-labeler#55

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (feec4e2) 85.70% compared to head (fe4d96a) 85.14%.
Report is 5 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3020      +/-   ##
===========================================
- Coverage    85.70%   85.14%   -0.57%     
===========================================
  Files          784      786       +2     
  Lines       126856   127351     +495     
===========================================
- Hits        108726   108435     -291     
- Misses       18130    18916     +786