feat: http sql api return schema on empty resultset

[sunng87]

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What's changed and what's your intention?

Previously we don't return schema for empty resultset. This patch corrected this for better usability.

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

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (814924f) 85.81% compared to head (80e5b1a) 85.37%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3237      +/-   ##
==========================================
- Coverage   85.81%   85.37%   -0.45%     
==========================================
  Files         845      845              
  Lines      138853   138876      +23     
==========================================
- Hits       119158   118561     -597     
- Misses      19695    20315     +620