Fix generate open api that occurs null value in generated file

[michelu89]

See #209

Description

Please include a summary of the changes and the related issue. List any dependencies that are required for this change.

Fixes #(number of issue in GitHub)

Type of change

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• 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 existing functionality to not work as expected)

Checklist:

• My code follows the style 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
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

Additional notes:

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