make sure we catch single character dbs and schemas

[dave-connors-3]

This is a:

• bug fix PR with no breaking changes
• new functionality

Link to Issue

Clsoes #454

Description & motivation

Makes sure we handle single character db.schema.table and schema.table patterns like

select * from a.b.c

and

select * from a.b

no adjustment select * from a to not affect detection of CTE names

Integration Test Screenshot

Checklist

• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
  • Databricks
  • DuckDB
  • Trino/Starburst
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)