[1.8.x] fix for mapping error status code for return nil type in Ballerina to OpenAPI spec generation

[lnash94]

Purpose

Fix ballerina-platform/ballerina-library#4924

Automation tests

• Unit tests - Added

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• Integration tests

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• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines? yes/no
• Ran FindSecurityBugs plugin and verified report? yes/no
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes/no

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (099183d) 79.64% compared to head (0847854) 79.70%.

Additional details and impacted files

@@             Coverage Diff              @@
##              1.8.x    #1535      +/-   ##
============================================
+ Coverage     79.64%   79.70%   +0.05%     
- Complexity     1768     1777       +9     
============================================
  Files           124      124              
  Lines         10082    10095      +13     
  Branches       1670     1673       +3     
============================================
+ Hits           8030     8046      +16     
+ Misses         1470     1464       -6     
- Partials        582      585       +3     

Files	Coverage Δ
...java/io/ballerina/openapi/converter/Constants.java	73.86% <ø> (ø)
...enapi/converter/service/OpenAPIResponseMapper.java	83.68% <100.00%> (+0.28%)	⬆️

... and 1 file with indirect coverage changes

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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[dilanSachi]

LGTM