🐛 📝 Fix batch get payment

[pcrespov]

What do these changes do?

• 🐛 Avoids calling gateway with empty body to avoid 422 response. This fixes an undesired side effect observed in the front-end during one-time-payment where the "pay" button wheel will just turn...
• 📝 Document openapi specs expected by the payments-gateway system.

Related issue/s

• 🐛 fixes bug in https://github.com/ITISFoundation/appmotion-exchange/issues/16
• fixes malfunctioning observed during One-time-payment in payment service #4657

How to test

cd services/payments
make install-dev
pytest  -vv -k test_payments_gateway_get_batch_with_no_items tests

Dev Checklist

• No ENV changes or I properly updated ENV (read the instruction)

DevOps

None

[codecov]

Codecov Report

Merging #5060 (baae6f8) into master (56fe30c) will decrease coverage by 24.3%.
The diff coverage is 100.0%.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #5060      +/-   ##
=========================================
- Coverage    85.2%   60.9%   -24.3%     
=========================================
  Files        1263     565     -698     
  Lines       51909   27780   -24129     
  Branches     1114     198     -916     
=========================================
- Hits        44261   16941   -27320     
- Misses       7415   10789    +3374     
+ Partials      233      50     -183     

Flag	Coverage Δ
integrationtests	59.6% <ø> (+16.0%)	⬆️
unittests	92.4% <100.0%> (+7.4%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...core_service_payments/services/payments_gateway.py	99.0% <100.0%> (+<0.1%)	⬆️

... and 1065 files with indirect coverage changes

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.4% Duplication

[sonarqubecloud]

Please retry analysis of this Pull-Request directly on SonarCloud.

[codeclimate]

Code Climate has analyzed commit baae6f8 and detected 0 issues on this pull request.

View more on Code Climate.