fix: make nullable as default when alter table

[yfractal]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Fix bug #364
Make nullable as default when alter table.

Please explain IN DETAIL what the changes are in this PR and why they are needed:

• Summarize your change (mandatory)
  Make nullable as default when alter table
• How does this PR work? Need a brief introduction for the changed logic (optional)
• Describe clearly one logical change and avoid lazy messages (optional)
• Describe any limitations of the current code (optional)

Checklist

• [] I have written the necessary rustdoc comments.
• [] I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #591 (f60bd38) into develop (f465040) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop     #591      +/-   ##
===========================================
+ Coverage    86.45%   86.47%   +0.01%     
===========================================
  Files          399      399              
  Lines        50640    50640              
===========================================
+ Hits         43783    43790       +7     
+ Misses        6857     6850       -7     

Flag	Coverage Δ
rust	86.47% <0.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sql/src/statements.rs	80.62% <0.00%> (ø)
src/servers/src/opentsdb.rs	98.41% <0.00%> (+1.58%)	⬆️
src/servers/src/error.rs	43.24% <0.00%> (+16.21%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[waynexia]

Looks good to me. Thanks for your contribution.

Could you please also add some unit tests to cover this scenario? Like asserting when the ColumnDef will be nullable and when not.

[killme2008]

LGTM

[yfractal]

Thanks every one.

I will add tests to cover this.