Bug: must specify both schema and database

[dlawin]

Describe the bug
When specifying the db for a schema-separated project, you still have to specify the prod_schema key in the dbt project config or else you get an error from

data-diff/data_diff/dbt.py

Lines 57 to 58 in 29bfa73

	if config_prod_database is None or config_prod_schema is None:
	raise ValueError("Expected a value for prod_database: or prod_schema: under \nvars:\n data_diff: ")

Describe the environment

> data-diff --version
v0.4.0

docker / Debian-bullseye / Python 3.10

[dlawin]

Sub issue of #386