fix(freshness): error on NULL loaded_at instead of reporting a ~56-year staleness

[kiwamizamurai]

Resolves #16489

Problem

When MAX(loaded_at_field) (or a loaded_at_query result) is SQL NULL — e.g. the source table has no rows — calculate_freshness_common read the value with PrimitiveArray::value(0). That accessor ignores the null bitmap and returns whatever is in the buffer slot, which is 0 (the Unix epoch). dbt source freshness therefore reported a specific-looking but meaningless staleness:

Stale source my_source.empty_table (last updated 56years 8months 26days 8h 2s ago)

Solution

Add a small helper, extract_non_null_nanos, that casts the column to nanoseconds (as before) and checks is_null(0) before calling .value(0). If the value is NULL it returns an explicit error instead of computing an age from garbage. It is used for both max_loaded_at and snapshotted_at (the latter defensively).

No public types, JSON artifacts, or CLI flags change.

Behavior of the error at the call sites (unchanged code, relying on the existing per-node handling in run_freshness):

• dbt source freshness: the node is reported as an error with the new message.
• dbt build / dbt run (freshness collected as part of state-aware orchestration): a warning is emitted and the source is treated as updated, so dependents rebuild. Previously an empty table was read as the epoch.

Alternative considered: a dedicated "no data" FreshnessStatus. I did not do it here because it would touch run_results / telemetry / artifacts; happy to follow up if maintainers prefer that direction.

Testing

Added unit tests for the helper (NULL → error; present value → correct nanoseconds, via a Timestamp(Microsecond, UTC) column as in the issue's reproduction).

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 or 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.