V5 - PgConnectionArgOrderByPlugin Regression - Error when passing null in orderBy variable

[Mattia Rossi]

Using V5, either in library mode or local server or in cli mode, the PgConnectionArgOrderByPlugin throws an error if passing an orderBy variable set as null:
(Using the Demo database from graphile/demo)
Query:

query MyQuery($first: Int, $orderBy: [UsersOrderBy!]) {
  users(first: $first, orderBy: $orderBy) {
    nodes {
      username
      authoredArticles {
        nodes {
          title
          url
        }
      }
    }
  }
}

Variables:

{
  "first": 5,
  "orderBy" :null
}

Error:

{
  "data": {
    "users": null
  },
  "errors": [
    {
      "message": "An error occurred (logged with hash: 'gI-xRchWPlyq39FMFlCQIfA2-dQ', id: '33LRV3HNCP')",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "users"
      ],
      "extensions": {
        "errorId": "33LRV3HNCP",
        "stack": [
          "Error: Invalid!",
          "    at /Users/postgraphile-v5-playground/node_modules/graphile-build-pg/dist/plugins/PgConnectionArgOrderByPlugin.js:126:15",
          "    at Object.plan [as applyPlan] (/Users/postgraphile-v5-playground/node_modules/graphile-build-pg/dist/plugins/PgConnectionArgOrderByPlugin.js:109:33)",
          "    at /Users/postgraphile-v5-playground/node_modules/grafast/dist/index.js:443:62698",
          "    at OperationPlan.withModifiers (/Users/postgraphile-v5-playground/node_modules/grafast/dist/index.js:225:21713)",
          "    at Object.apply (/Users/postgraphile-v5-playground/node_modules/grafast/dist/index.js:443:62555)",
          "    at d (/Users/postgraphile-v5-playground/node_modules/grafast/dist/index.js:443:59177)",
          "    at t.withFieldArgsForArguments (/Users/postgraphile-v5-playground/node_modules/grafast/dist/index.js:443:65092)",
          "    at t.withGlobalLayerPlan (/Users/postgraphile-v5-playground/node_modules/grafast/dist/index.js:443:16675)",
          "    at OperationPlan.planField (/Users/postgraphile-v5-playground/node_modules/grafast/dist/index.js:225:20211)",
          "    at OperationPlan.processGroupedFieldSet (/Users/postgraphile-v5-playground/node_modules/grafast/dist/index.js:225:14506)"
        ]
      }
    }
  ]
}