fix: Prevent exception in checkForDefinition if parameter has no 'name'

[hkosova]

Description

During semantic validation, this line
https://github.com/ralphdoe/swagger-editor/blob/master/src/plugins/validate-semantic/validators/helpers.js#L42
tries to convert the parameter name to lower case. But if the parameter has no name, then toLowerCase() will raise an exception.

This PR attempts to fix the issue.

Motivation and Context

Fixes #2161.

How Has This Been Tested?

Manually.

I don't know how to check this in unit tests.

Screenshots (if appropriate):

n/a

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.