SF-3662 Ensure that no books are trained on when none are selected

[pmachapman]

This PR fixes a bug where Scripture Forge should send an empty string to Serval to not train on any books, when no books were selected by the user.

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This change is 

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.76%. Comparing base (c4a8e75) to head (67024cf).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...XForge.Scripture/Services/MachineProjectService.cs	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3606      +/-   ##
==========================================
- Coverage   82.77%   82.76%   -0.01%     
==========================================
  Files         610      610              
  Lines       37235    37236       +1     
  Branches     6080     6105      +25     
==========================================
  Hits        30820    30820              
+ Misses       5498     5485      -13     
- Partials      917      931      +14     

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[RaymondLuong3]

@RaymondLuong3 reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @pmachapman)