fix default for BiblicalTermsListSetting

[mshannon-sil]

The reason that BiblicalTermsListSetting wasn't allowed to be optional was because the default was set to "", but the check to see if a BiblicalTermsListSetting exists in the Settings.xml uses if biblical_terms_list_setting is not None. I've removed the explicit default so that if it's not found, it assigns None, and the subsequent check will work properly. This bug is only in machine.py, not in Machine.

I tested this by temporarily removing the BiblicalTermsListSetting line in the Settings.xml of the Tes folder, and then verifying that the change in defaults successfully changed tests from failing to passing. However, there were a couple tests that depended on the contents of BiblicalTermsListSetting not being empty/default, so I had to undo that change in Settings.xml when committing.

Should I create test case(s) for the parse() method in ParatextProjectSettingsParserBase along with this bug fix? I'd imagine that would require another test file since there isn't one for ParatextProjectSettingsParserBase, and then adding a corresponding test file in Machine as well to keep the repos matching.

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This change is 

[ddaspit]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @mshannon-sil)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.14%. Comparing base (7a2123f) to head (e2b3206).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #125   +/-   ##
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  Coverage   88.14%   88.14%           
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  Files         273      273           
  Lines       16010    16010           
=======================================
  Hits        14112    14112           
  Misses       1898     1898           

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