fix: include zero-rated vat transactions in vat return

[dvdl16]

Description

• fix: include zero-rated vat transactions in vat return

Fixes #33

Type of change

• 🟢 Bug fix (change which fixes an issue)
• ⚪ New feature (change which adds functionality)
• ⚪ Breaking change (fix or feature that would cause existing functionality to not work as expected)

Tests

• Unit Tests have been updated or added, as required
• UI Tests have been updated or added, as required

Checklist:

• My code follows Naming Guidelines (DocType, Field and Variable naming)
• No Form changes / My code follows the Form Design Guidelines
• My code follows the Coding Standards of this project
• My code follows the Code Security Guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas / No comments necessary
• I have made corresponding additions/changes to the documentation
• All business logic and validations are on the server-side / No business logic or validation changes
• No patches are necessary / Migration Patches have been added to the correct subdirectory of /patches and patches.txt have been updated

User Experience:

N/A

[codecov-commenter]

Codecov Report

❌ Patch coverage is 42.85714% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.16%. Comparing base (7b63994) to head (0c7ea7e).

Files with missing lines	Patch %	Lines
...e/value_added_tax_return/value_added_tax_return.py	46.15%	7 Missing ⚠️
csf_za/__init__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           version-15      #34      +/-   ##
==============================================
- Coverage       40.30%   40.16%   -0.14%     
==============================================
  Files              10       10              
  Lines             464      473       +9     
==============================================
+ Hits              187      190       +3     
- Misses            277      283       +6     

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