fix(form): avoid undefined message error as string

[k8adev]

Add validation to prevent converting undefined error messages to string in the FormMessage component.

This fix addresses issues that occur in complex validation structures, such as validating entire objects.

const formSchema = z.object({
  location: z.object({
    city: z.string().nonempty(),
    state: z.string().nonempty(),
    country: z.string().nonempty(),
  }).superRefine((data, ctx) => {
    ...
  });

<FormField
  control={form.control}
  name="location"
  render={() => (
    <FormItem>
      <FormField
        control={form.control}
        name="location.city"
        render={({ field }) => (
          <FormItem>
            <FormLabel>City</FormLabel>
            <FormControl>
              <Input placeholder="Enter city" {...field} />
            </FormControl>
          </FormItem>
        )}
      />

      <FormField
        control={form.control}
        name="location.state"
        render={({ field }) => (
          <FormItem>
            <FormLabel>State</FormLabel>
            <FormControl>
              <Input placeholder="Enter state" {...field} />
            </FormControl>
          </FormItem>
        )}
      />

      <FormField
        control={form.control}
        name="location.country"
        render={({ field }) => (
          <FormItem>
            <FormLabel>Country</FormLabel>
            <FormControl>
              <Input placeholder="Enter country" {...field} />
            </FormControl>
          </FormItem>
        )}
      />

      <FormMessage />
    </FormItem>
  )}
/>

[vercel]

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