Treat empty nullable CSV fields as NULL during sync

[Copilot]

Empty CSV cells in nullable typed columns were not handled consistently: SQLite stored "", while PostgreSQL rejected numeric empties before they could become NULL. This updates sync behavior so nullable: true is sufficient on its own and lookup: { "": null } is no longer required for ordinary empty-field handling.

• Behavior change

  • Convert empty CSV cells to NULL for nullable columns after lookup processing
  • Apply this consistently across configured types, including integer, bigint, float, double, boolean, date, datetime, timestamp, text, string, and varchar

• Lookup interaction

  • Preserve explicit lookup results
  • Only treat values as NULL when the final transformed value is still empty, so non-empty replacements from lookup continue to work as before

• Regression coverage

  • Add integration coverage for empty nullable values across both SQLite and PostgreSQL
  • Cover all supported configured data types to prevent backend- or type-specific regressions

• Versioning

  • Bump the package version from 0.7.0 to 0.7.1

columns:
  density:
    db_column: density
    type: float
    nullable: true

With this change, an empty density field in the CSV now syncs as database NULL without requiring:

lookup:
  "": null

[github-actions]

Test Results (Linux 3.14)

514 tests  +2   514 ✅ +2   39s ⏱️ +2s
  1 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit cadc6a9. ± Comparison against base commit 486a80a.

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
src/crump
config.py					1047
Project Total

This report was generated by python-coverage-comment-action

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.03%. Comparing base (486a80a) to head (cadc6a9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/crump/config.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #78      +/-   ##
==========================================
+ Coverage   88.00%   88.03%   +0.03%     
==========================================
  Files          17       17              
  Lines        2759     2759              
==========================================
+ Hits         2428     2429       +1     
+ Misses        331      330       -1     

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