Postgraphile collection queries erroring when using 'mergeSchemas'
I have the following code: 

versions: 
```
    "postgraphile": "^4.12.3",
    "postgraphile-apollo-server": "^0.1.1",
    "postgraphile-plugin-connection-filter": "^2.2.1",

    // `mergeSchemas`: i'm aware if it's an issue here, this is not your problem, but thought I'd include it to be exhaustive
    "graphql-tools": "^7.0.5",
```

```javascript
export const getGraphqlSchema = async () => {
  const { schema: pgSchema, plugin: PgPlugin } = await getPgSchema();
  const customSchema = getCustomSchema();

  const schema = mergeSchemas({
    schemas: [pgSchema, customSchema],
  });
  const schemaWithMiddleware = applyMiddleware(schema, ...middlewares);

  return {
    schema: schemaWithMiddleware,
    plugins: [PgPlugin],
    formatResponse,
    context,
    middleware: [],
  };
};
```

Where `customSchema` is using `makeExecutableSchema` from `apollo-server-express` and pg schema instantiation looks like this: 

```javascript
export const getPgSchema = async () => {
  const { pool } = await setupConnection();

  return makeSchemaAndPlugin(
    pool,
    'public', // PostgreSQL schema to use
    {
      // PostGraphile options, see:
      // https://www.graphile.org/postgraphile/usage-library/

      // require("@graphile-contrib/pg-many-to-many"),
      appendPlugins: [require('@graphile-contrib/pg-simplify-inflector')],
    },
  );
};
```

This strategy works for everything except collection queries. All other queries/mutations from both postgraphile and custom schemas work perfectly. If I try to run something like:

```
query GetUsers {
  users {
    nodes {
      id
    }
  }
}
```

I get this error:

```json
{
  "errors": [
    {
      "message": "Cannot read property 'map' of undefined",
      "locations": [
        {
          "line": 3,
          "column": 5
        }
      ],
      "path": [
        "users",
        "nodes"
      ],
      "extensions": {
        "code": "INTERNAL_SERVER_ERROR",
        "exception": {
          "stacktrace": [
            "TypeError: Cannot read property 'map' of undefined",
            "    at resolve (/Users/daniellemburg/Desktop/made/made-web-api/node_modules/graphile-build-pg/src/plugins/PgTablesPlugin.js:501:44)",
            "    at field.resolve (/Users/daniellemburg/Desktop/made/made-web-api/node_modules/apollo-server-core/src/utils/schemaInstrumentation.ts:103:18)",
            "    at resolveField (/Users/daniellemburg/Desktop/made/made-web-api/node_modules/graphql/execution/execute.js:464:18)",
            "    at executeFields (/Users/daniellemburg/Desktop/made/made-web-api/node_modules/graphql/execution/execute.js:292:18)",
            "    at collectAndExecuteSubfields (/Users/daniellemburg/Desktop/made/made-web-api/node_modules/graphql/execution/execute.js:748:10)",
            "    at completeObjectValue (/Users/daniellemburg/Desktop/made/made-web-api/node_modules/graphql/execution/execute.js:738:10)",
            "    at completeValue (/Users/daniellemburg/Desktop/made/made-web-api/node_modules/graphql/execution/execute.js:590:12)",
            "    at /Users/daniellemburg/Desktop/made/made-web-api/node_modules/graphql/execution/execute.js:469:16",
            "    at processTicksAndRejections (node:internal/process/task_queues:96:5)",
            "    at async Promise.all (index 0)"
          ]
        }
      }
    }
  ],
  "data": {
    "users": null
  }
}
```

I stepped through the resolver, and when it gets ready to execute the sql against the pg client, the query itself is just this: ` select `. Naturally, this does not return any results, haha.

Note that I am using `@graphile-contrib/pg-simplify-inflector` but the same error happens regardless of using this plugin.


All collection queries work fine when I do not use `mergeSchemas`. Any thoughts?

Thanks!


