Incorrect typing of idColumn

[jackbonaguro]

According to documentation, idColumn should be set to null in the event a table does not have an id column: https://vincit.github.io/objection.js/api/model/static-properties.html#static-idcolumn

But due to the typings, null is not a possible valid value for idColumn - only string | string[]:

objection.js/typings/objection/index.d.ts

Line 1541 in d1e194a

static idColumn: string | string[];

This causes an issue in my code on version 3.1.4. Doing either:

static idColumn = null;

or

static get idColumn() {
    return null;
}

Results in a compiler error:

  Types of property 'idColumn' are incompatible.
    Type 'null' is not assignable to type 'string | string[]```

Only by casting like `static idColumn = null as unknown as string;` can I work around the issue. Not setting idColumn causes runtime errors.

[lehni]

Thanks for pointing this out @jackbonaguro. I'm happy to accept a PR that fixes this.