Implement -fixEmptyArrays to force empty array [] instead of null in JSON

[VojtechVitek]

Golang issue: golang/go#27589

This option will enforce server to encode nil slices as empty arrays in JSON.

This is helpful mainly in JavaScript and TypeScript clients, where a response field of type array should never be assigned to null value..

Example:

func (s *Server) GetItems(ctx context.Context) ([]*Item, error) {
	var items []*Item

	// Will send {"items": []} in JSON response instead of {"items": null}.
	return items, nil
}