chore: add undefined type to principalAccount (#33055)

### Reason for this change


When using TypeScript option exactOptionalPropertyTypes: true` we get the following error. All implementations of `IPrincipal` have the type `string | undefined` so this is change will apply it at the base

```
Type 'AccountPrincipal' is not assignable to type 'IPrincipal' with 'exactOptionalPropertyTypes: true'. Consider adding 'undefined' to the types of the target's properties.
  Types of property 'principalAccount' are incompatible.
    Type 'string | undefined' is not assignable to type 'string'.
      Type 'undefined' is not assignable to type 'string'.ts(2375)
```

### Description of changes

Changed type definition of IPrincipal `principalAccount`

### Describe any new or updated permissions being added
N/A

### Description of how you validated changes



### Checklist
- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: dali546

packages/aws-cdk-lib/aws-iam/lib/principals.ts

@@ -53,7 +53,7 @@ export interface IPrincipal extends IGrantable {
    * Can be a Token - in that case,
    * it's assumed to be AWS::AccountId.
    */
-  readonly principalAccount?: string;
+  readonly principalAccount?: string | undefined;
 
   /**
    * Add to the policy of this principal.