Title: Collector fails on long_running_transactions when oldest_timestamp_seconds is NULL

[alex-ganin]

Description
The collector fails with the following error when there are no long-running transactions in the database:

collector failed: sql: Scan error on column index 1, name "oldest_timestamp_seconds": converting NULL to float64 is unsupported

Steps to Reproduce

Set up the collector on a PostgreSQL database.

Ensure the long_running_transactions metric is queried when there are no long-running transactions (the oldest_timestamp_seconds column is NULL).

Run the collector.

Expected Behavior
The collector should handle NULL values gracefully, e.g., by treating them as 0 or skipping the metric.

Actual Behavior
Collector crashes due to sql: Scan error when trying to assign NULL to float64.

Environment:
PostgreSQL version: 17.6
postgres-exporter:v0.18.1 (docker image)