Fix empty 'save()' as query last statement

[dreadatour]

Fix for #327
See this comment.

Closes #343.

[cloudflare-workers-and-pages]

Deploying datachain-documentation with    Cloudflare Pages

Latest commit:	fe75445
Status:	✅  Deploy successful!
Preview URL:	https://60d3addb.datachain-documentation.pages.dev
Branch Preview URL:	https://fix-empty-save-in-query.datachain-documentation.pages.dev

View logs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.90%. Comparing base (f0db01d) to head (fe75445).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #357   +/-   ##
=======================================
  Coverage   86.89%   86.90%           
=======================================
  Files          90       90           
  Lines        9923     9925    +2     
  Branches     2003     2003           
=======================================
+ Hits         8623     8625    +2     
  Misses        947      947           
  Partials      353      353           

Flag	Coverage Δ
datachain	86.83% <100.00%> (+<0.01%)	⬆️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.