feat: add support for nullable response types

[asadali214]

What

This PR adds complete support for returning nullable response types using the nullableType() builder function to set the response types as nullable.

Why

Previously response handler was throwing an exception or returning '' when a response with no content is received.

Closes #60

Type of change

Select multiple if applicable.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause a breaking change)
• Tests (adds or updates tests)
• Documentation (adds or updates documentation)
• Refactor (style improvements, performance improvements, code refactoring)
• Revert (reverts a commit)
• CI/Build (adds or updates a script, change in external dependencies)

Dependency Change

N/A

Breaking change

N/A

Testing

Unit tests have been added to test this feature

Checklist

• My code follows the coding conventions
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added new unit tests

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
2.7% Duplication on New Code

See analysis details on SonarCloud