Fixes for empty SQL sub-objects

[milessabin]

The handling of empty SQL subobjects (previously represented by a context-free singleton EmptySqlQuery) was incorrect in several scenarios.

This showed up in particular in mixed mappings where non-SQL fields were nested in SQL objects. This manifested itself as unexpectedly empty arrays of subobjects, or possibly hit previously unimplemented methods of EmptySqlQuery. Both are fixed by ensuring that SQL ids are included when we, in effect, "join" a SQL with a non-SQL object. EmptySqlQuery is now a non-singleton and has a context and the unimplemented methods have been either implemented or removed.

[codecov-commenter]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (b5b703c) 72.39% compared to head (63f8368) 72.48%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
modules/sql/shared/src/main/scala/SqlMapping.scala	84.15%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #519      +/-   ##
==========================================
+ Coverage   72.39%   72.48%   +0.08%     
==========================================
  Files          32       32              
  Lines        4340     4332       -8     
  Branches      927      951      +24     
==========================================
- Hits         3142     3140       -2     
+ Misses       1198     1192       -6     

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

[swalker2m]

Thanks for working on this. I tested it with my code that had been failing and it worked great. 👍