feat: Implement a configuration option for the initial "send empty value" checkbox state.

[LincolnPuzey]

Description

This new configuration option defaultSendEmptyValue / DEFAULT_SEND_EMPTY_VAlUE can be used to set the initial state of the "send empty value" checkbox. Default is true (checked).

Motivation and Context

Fixes #6505

In our API, we have lots of multipart form fields which are allowed to be ommitted, but when provided must follow some pattern (e.g. valid Date, URL, email, etc).
Currently these are all sent as empty strings ("Send empty value" is checked) by default,
which causes lots of errors.
Users interacting with the form must un-check these fields when not providing a value.
Being able to set "Send empty value" to un-checked (false) by default, results in a much better user experience.

How Has This Been Tested?

I have built swagger-ui locally with my changes and swapped swagger-ui-bundle.js into our project to test it.

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.

Re tests: I have updated the docker config tests. Not sure if any more tests are required.