critical error when using nullable enum

### What are you generating using Kiota, clients or plugins?

API Client/SDK

### In what context or format are you using Kiota?

Windows executable

### Client library/SDK language

TypeScript

### Describe the bug

So I get critical error when trying to build with this enum in spec

```
"Currency": {
"enum": [
"USD",
"GBP",
"EUR",
null
]
}
```

```
"currency": {
"oneOf": [
{
"type": "null"
},
{
"$ref": "#/components/schemas/Currency"
}
]
}
```

### Expected behavior

generate client without any problem

### How to reproduce

use nullable enum somewhere in code

### Open API description file

_No response_

### Kiota Version

1.29.0

### Latest Kiota version known to work for scenario above?(Not required)

_No response_

### Known Workarounds

_No response_

### Configuration

_No response_

### Debug output

Click to expand log
```

```

### Other information

_No response_

No source file, test, or entry point is identified in the report. Start by reproducing generation with the shown nullable enum in Kiota 1.29.0, capture the critical error, and trace the OpenAPI enum/null handling; done means a TypeScript client generates successfully without the critical error.

Written by the indexing model from the issue text.