[Regression] docs generate raises error when querying external tables

[mikealfare]

Is this a regression in a recent version of dbt-bigquery?

• I believe this is a regression in dbt-bigquery functionality
• I have searched the existing issues, and I could not find an existing issue for this regression

Current Behavior

Creating a model dependent on an external table and running dbt docs generate raises this error:

File "/venv/dbt-1.7.0-latest/lib/python3.8/site-packages/dbt/task/generate.py", line 106, in add_column
    column_data["index"] = int(column_data["index"])
TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'

Expected/Previous Behavior

No error should be raised and the docs should include this source, but with unknown columns, as was previously the case in 1.7.2

Steps To Reproduce

1. Create a model dependent on an external table
2. Run dbt docs generate

Relevant log output

No response

Environment

- OS: dbt Cloud
- Python: 3.8
- dbt-core (working version): 1.7
- dbt-bigquery (working version): 1.7.2
- dbt-core (regression version): 1.7
- dbt-bigquery (regression version): 1.7.3

Additional Context

I think we need to coalesce here:

dbt-bigquery/dbt/include/bigquery/macros/catalog/catalog.sql

Lines 124 to 126 in 8b98265

	columns.column_name,
	columns.column_index,
	columns.column_type,

to match (note the comment):

dbt-bigquery/dbt/include/bigquery/macros/catalog.sql

Lines 171 to 180 in b160b05

	-- coalesce name and type for External tables - these columns are not
	-- present in the COLUMN_FIELD_PATHS resultset
	coalesce(columns.column_name, '<unknown>') as column_name,
	-- invent a row number to account for nested fields -- BQ does
	-- not treat these nested properties as independent fields
	row_number() over (
	partition by relation_id
	order by columns.column_index, columns.column_name
	) as column_index,
	coalesce(columns.column_type, '<unknown>') as column_type,