Fix hanging dbt source freshness command when loaded_at_field is omitted

[mikealfare]

resolves #1044

Problem

When loaded_at_field is omitted from a source, dbt source freshness has unexpected behavior. The command does not return. If loaded_at_field is provided and nothing else is changed, the command returns.

Solution

• Provide a test case that addresses the issue
• Raise a warning when loaded_at_field is missing
• Skip the source when loaded_at_field is missing

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

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the dbt-bigquery contributing guide.