feat: add preserveEmptyObjects filter option + remove keepEmptySchema option

[mamatsunami]

Having a dedicated option--keepEmptySchema to prevent removal of empty schema object wasn't flexible enough.

I decided to completely remove it and instead I added a preserveEmptyObjects which is a filtering option.

It can be :

• undefined : we keep original behavior (removal of all empty objects except for examples)
• false : we remove all (even examples)
• true : we keep all
• [...] : for instance with ['examples', 'schema'] value, we preserve all empty objects defined inside examples and schema paths.

[thim81]

@mamatsunami this is a great addition for the filtering option. It suits better than making it a separate option. Thanks again for taking the time and effort to provide a ready to merge PR.

[thim81]

hi @mamatsunami,

Just wanted to let you know, that there is a new release of OpenAPI format which contains your PR.
I wanted to thank you for your contribution.