[BUG] spanner errors out when spanner_config does not include credentials file

[DataBoyTX]

Describe the bug
There are 2 methods for authenticating with Spanner:

1. gcp service account credentials file, and
2. web-based interactive authentication

a new optional parameter was added to register() to allow passing in spanner config:

        spanner_config dict must contain the include the following keys, credentials_file is optional:
            - "project_id": The GCP project ID.
            - "instance_id": The Spanner instance ID.
            - "database_id": The Spanner database ID.
            - "credentials_file": json file API key for service accounts 

if using method #2 (web-based interactive auth) which does not require credentials_file, spanner_init errors with:

SpannerConnectionError: Failed to connect to Spanner: 'SpannerGraph' object has no attribute 'credentials_file'

To Reproduce

use option 2, which does not require credentials_file to be set, then attempt to call

import pandas as pd
import graphistry

PROJECT_ID   = "my_project" 
INSTANCE_ID  = "my_instance" 
DATABASE_ID  = "finance-graph-db"  

# google settings, option 1: interactive login using gcloud auth application-default login (below)
SPANNER_CONF = { "project_id":  PROJECT_ID, 
                 "instance_id": INSTANCE_ID, 
                 "database_id": DATABASE_ID }


graphistry.register(api=3, personal_key_id='xxx', personal_key_secret='xxx', server='hub.graphistry.com', spanner_config=SPANNER_CONF) 

# google auth stuff: 

# Set the google project id
!gcloud config set project {PROJECT_ID}
%env GOOGLE_CLOUD_PROJECT={PROJECT_ID}

# web-based interactive login to GCP: 
!gcloud auth application-default login

# spanner graph query: 

query=f'''GRAPH FinGraph
MATCH p = (a)-[b]->(c) where 1=1 {LIMIT_CLAUSE} return SAFE_TO_JSON(p) as path'''

g = graphistry.spanner_gql_to_g(query)

Expected behavior
should not error out as credentials_file is not required with web-based auth

Actual behavior

AttributeError                            Traceback (most recent call last)
/usr/local/lib/python3.11/dist-packages/graphistry/plugins/spannergraph.py in __connect(self)
     84         try:
---> 85             if self.credentials_file:
     86                 connection = connect(self.instance_id, self.database_id, credentials=self.credentials_file)

AttributeError: 'SpannerGraph' object has no attribute 'credentials_file'

During handling of the above exception, another exception occurred:

SpannerConnectionError                    Traceback (most recent call last)
5 frames
/usr/local/lib/python3.11/dist-packages/graphistry/plugins/spannergraph.py in __connect(self)
     92             return connection
     93         except Exception as e:
---> 94             raise SpannerConnectionError(f"Failed to connect to Spanner: {e}")
     95 
     96     def close_connection(self) -> None:

SpannerConnectionError: Failed to connect to Spanner: 'SpannerGraph' object has no attribute 'credentials_file'