feat: add isnull function

[KKould]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Implement the isnull function to find remainders

• added a unit test
• updated the sqlness test

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

issue: #3355

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (53105b9) 85.38% compared to head (b8c672c) 84.97%.
Report is 3 commits behind head on main.

❗ Current head b8c672c differs from pull request most recent head a0686de. Consider uploading reports for the commit a0686de to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3360      +/-   ##
==========================================
- Coverage   85.38%   84.97%   -0.41%     
==========================================
  Files         889      891       +2     
  Lines      145477   145534      +57     
==========================================
- Hits       124210   123662     -548     
- Misses      21267    21872     +605     

[waynexia]

Others LGTM, appreciate it @KKould 🚀

[tisonkun]

Thanks for your contribution!

[tisonkun]

@KKould out of curiosity, and since there is no linked good first issue, I wonder how you found this project :D

And welcome! You can browse other good first issue or start a discussion at the Discussions forum for anything you are interested in this project.

[KKould]

@KKould out of curiosity, and since there is no linked good first issue, I wonder how you found this project :D

And welcome! You can browse other good first issue or start a discussion at the Discussions forum for anything you are interested in this project.

I was a database employee and followed Greptime for a long time, but I am more interested in things related to SQL queries. I have not encountered many related issues before.