fix(core): keep empty object on json serialization

[tchiotludo]

[brian-mulier-p]

See this fix https://github.com/kestra-io/plugin-aws/pull/203 that should be removed and replaced with Collections.emptyList once we serialize empty objects properly. Note that due to the fact that when triggering an execution we transform the Flow to a Map then back to a Flow, @JsonInclude on a specific field has no effect

[loicmathieu]

Closed in favor of https://github.com/kestra-io/kestra/pull/2358