Optional null field results in Zod schema syntax error

[quintal-william]

What version of kubb is running?

3.6.1

What kind of platform do you use?

Linux

How does your kubb.config.ts config look like

import { defineConfig } from '@kubb/core'
import { pluginOas } from '@kubb/plugin-oas'
import { pluginZod } from '@kubb/plugin-zod'

export default defineConfig({
  name: 'Test',
  root: './src',
  input: {
    data: JSON.stringify({
      openapi: '3.1.0',
      components: {
        schemas: {
          TestSchema: {
            properties: {
              stringField: {
                type: 'string',
              },
              nullField: {
                type: 'null',
              },
            },
            type: 'object',
            required: [],
          },
        },
      },
    }),
  },
  output: {
    path: './__generated__',
    clean: true,
  },
  plugins: [pluginOas(), pluginZod()],
})

What version of external packages are you using(@tanstack-query, MSW, React, Vue, ...)

{
  "zod": "3.24.2",
  "react": "19.0.0",
  "typescript": "5.7.3"
}

What steps can reproduce the bug?

Given the example kubb.config.ts file, run the generator and observe the syntax error in the generated zod schema:

import { z } from "zod";

export const testSchemaSchema = z.object({
      "stringField": z.string().optional(),
  "nullField": .optional()z.null()
      })

How often does this bug happen?

Every time

What is the expected behavior?

The null field should match the behaviour of the string field

import { z } from "zod";

export const testSchemaSchema = z.object({
    "stringField": z.string().optional(),
    "nullField": z.null().optional()
})

Additional information

No response