Redshift: fix object types with empty properties getting ignored

[chuwy]

If we process following schema with igluctl static generate:

{
  "$schema": "http://iglucentral.com/schemas/com.snowplowanalytics.self-desc/schema/jsonschema/1-0-0#",
  "self": {
    "vendor": "com.acme",
    "name": "site_search",
    "format": "jsonschema",
    "version": "1-0-0"
  },
  "description": "Schema for search interaction events with filters as arrays",
  "properties": {
    "filters": {
      "description": "Active site search filters",
      "type": [ "object", "null" ],
      "properties": {},
      "additionalProperties": true
    },
    "sorting": {
      "description": "Active site search sorting",
      "type": [ "object", "null" ],
      "properties": {},
      "additionalProperties": true
    }
  },
  "additionalProperties": false,
  "type": "object"
}

In resulting DDL there will be no filters nor sorting columns. If we remove properties from both fields - they're getting added.

[chuwy]

As mentioned in snowplow/iglu-central#1137 the behavior is different if we have additionalProperties with object in it, but I belive the root of the bug is the same.