When `last_customer_id` is None, `last_customer_id` is converted to the string "None"

[ni4shi4]

Thank you for your continued work on this library.

Describe the bug:
When login_customer_id is set to None, the library converts it to the string "None" (via str(...)) during config validation, which then fails customer ID validation.

This appears to happen in the config validation flow (related code):

• google/ads/googleads/config.py around validate_dict / validate_login_customer_id

As a result, passing login_customer_id=None raises a ValueError even though the user intent is “unset / not provided”.

Steps to Reproduce:

1. Set the credentials as below (sanitized values; only login_customer_id=None matters)

credentials = {
    "developer_token": "REDACTED",
    "client_id": "REDACTED",
    "client_secret": "REDACTED",
    "refresh_token": "REDACTED",
    "login_customer_id": None,
    "use_proto_plus": True,
}

2. Initialize the client using the credentials

from google.ads.googleads.client import GoogleAdsClient

client = GoogleAdsClient.load_from_dict(credentials, version="v20")

Expected behavior:
No error (or login_customer_id remains None / is treated as “not set”), so that initialization succeeds when login_customer_id is intentionally omitted.

Client library version and API version:
Client library version: v29.0
Google Ads API version: v20

Request/Response Logs:

...
  File "xxxx.py", line xxxx, in xxxx
    client = GoogleAdsClient.load_from_dict(credentials,version="v20")
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/google/ads/googleads/client.py", line 291, in load_from_dict
    config_data: Dict[str, Any] = config.load_from_dict(config_dict)
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/google/ads/googleads/config.py", line 77, in validation_wrapper
    validate_dict(config_dict)
  File "/usr/local/lib/python3.12/site-packages/google/ads/googleads/config.py", line 208, in validate_dict
    validate_login_customer_id(str(config_data["login_customer_id"]))
  File "/usr/local/lib/python3.12/site-packages/google/ads/googleads/config.py", line 244, in validate_login_customer_id
    _validate_customer_id(login_customer_id, "login")
  File "/usr/local/lib/python3.12/site-packages/google/ads/googleads/config.py", line 230, in _validate_customer_id
    raise ValueError(
ValueError: The specified login customer ID is invalid. It must be a ten digit number represented as a string, i.e. '1234567890'
)

Anything else we should know about your project / environment: