StringType enum option allows passing "undefined" and undefined

ottoman version: 2.5.2

[StringType](https://ottomanjs.com/docs/api/classes/StringType) enum option allows passing `"undefined"` and `undefined`.

It also sets the value to `"undefined"`.

When constructing the document, `undefined` will be cast to `"undefined"`.

When saving the document, `"undefined"` will pass the check here:

https://github.com/couchbaselabs/node-ottoman/blob/f205b2c60146abe04165e78659c16ce0d15a4164/src/schema/types/string-type.ts#L262

example:

```js
import { connect, model, Schema } from "ottoman";

await connect({
bucketName: "travel-sample",
connectionString: "couchbase://localhost",
username: "Administrator",
password: "password",
});

const userSchema = new Schema({
gender: { type: String, enum: ["M", "F"] },
});

const User = model("User", userSchema, {
scopeName: "_default",
collectionName: "_default",
});

const a = new User({ gender: undefined });
await a.save();
console.log(a.gender === "undefined"); // got true, expected false

const aa = await User.findById(a.id);
console.log(aa.gender === "undefined"); // got true, expected false

const b = new User({ gender: "undefined" });
await b.save(); // passes, expected ValidationError
console.log(b.gender === "undefined"); // got true, expected false

const bb = await User.findById(b.id);
console.log(bb.gender === "undefined"); // got true, expected false
```

Start in src/schema/types/string-type.ts at the validation logic linked in the report, and reproduce the provided constructor and save examples. Done means undefined is not converted to the string "undefined", and the literal "undefined" is rejected for the enum option with the expected validation behavior.

Written by the indexing model from the issue text.