Typescript undefined type is not supported by schema type utilities

When trying to define a Schema where one property is optional (undefined) and not `null` in Typescript I'm getting type errors and cannot build the typescript. I know the error states " 'nullable' is missing in type" and I have no interest treating null and undefined the same, I want only the object to be valid if it has the property of type string or not defined in the object so that is in line white the Typescript object.

**What version of Ajv are you using? Does the issue happen if you use the latest version?**

v8.11.0

**Ajv options object**

```typescript
interface TheType {
key2: string | undefined;
}
const schema: JSONSchemaType = {
additionalProperties: true,
required: [],
properties: {
key2: { type: "string" },
},
type: "object",
}
```

**The error**

```
Type '{ additionalProperties: true; required: never[]; properties:
{ key2: { type: "string"; }; }; type: "object"; }' is not assignable to
type 'UncheckedJSONSchemaType'.
The types of 'properties.key2' are incompatible between these types.
Type '{ type: "string"; }' is not assignable to type '{ $ref: string; }
| (UncheckedJSONSchemaType
& { nullable: true; const?: null | undefined;
enum?: readonly (string | null | undefined)[]
| undefined; default?: string | ... 1 more ... | undefined; })'.
Type '{ type: "string"; }' is not assignable to type '{ type: "string"; }
& StringKeywords & {
allOf?: readonly UncheckedPartialSchema[]
| undefined; anyOf?: readonly UncheckedPartialSchema[]
| undefined; ... 4 more ...; not?: UncheckedPartialSchema<...>
| undefined; } & { ...; } & { ...; }'.
Property 'nullable' is missing in type '{ type: "string"; }'
but required in type '{ nullable: true; const?: null | undefined;
enum?: readonly (string | null | undefined)[]
| undefined; default?: string | null | undefined; }'.
```

** **

```typescript
interface TheType {
key2: string | undefined;
}
const schema: JSONSchemaType = {
additionalProperties: true,
required: [],
properties: {
key2: { nullable: false, type: "string" },
},
type: "object",
}
```

**The error**

```
Type '{ additionalProperties: true; required: never[]; properties: { key2: {
nullable: false; type: "string"; }; }; type: "object"; }'
is not assignable to type 'UncheckedJSONSchemaType'.
The types of 'properties.key2' are incompatible between these types.
Type '{ nullable: false; type: "string"; }' is not assignable to
type '{ $ref: string; } | (UncheckedJSONSchemaType
& { nullable: true; const?: null | undefined;
enum?: readonly (string | null | undefined)[] | undefined;
default?: string | ... 1 more ... | undefined; })'.
Types of property 'nullable' are incompatible.
Type 'false' is not assignable to type 'true'.

```