Fix null column index issue during dbt docs generate for external tables

[mikealfare]

resolves #1079

Problem

External tables don't show up in COLUMN_FIELD_PATHS metadata. When generating the catalog, these columns were returning null values for external tables. dbt docs generate does not account for null values for these fields, in particular for the column index.

Solution

Coalesce the queried values with default values:

• column_name: '<unknown>'
• column_index: 1
• column_type: '<unknown>'
• column_comment: ''

The '<unknown>' values align with the previous implementation, as does the 1. In the previous implementation, this was implicit, as it was calculated as a row_number across null fields. In this implementation, it's explicitly 1, as there is really only one field, "the data". The column comment in the original implementation was null in this scenario; this is an attempt to avoid such an error with this field in the future.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX