allowEmptyValue controls

[shockey]

Description

This PR adds a UI control for whether parameters with allowEmptyValue: true should be omitted or sent empty.

Motivation and Context

Fixes #1934, fixes #4223, and fixes #4587.

How Has This Been Tested?

I added unit and e2e tests where relevant 😄

Screenshots (if appropriate):

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.

[heldersepu]

Since this change adds a new param I think the documentation should also change...
https://github.com/swagger-api/swagger-ui/blob/master/docs/usage/configuration.md