Azure Billing integration return 0 value for 0 costed integration

*Description:

Currently, the Elastic Azure Billing integration does not return any data if a subscription has a 0 cost in the current month. This results in missing data points in dashboards, making it difficult to track months with no spending.

*Enhancement Request:

Ensure that the integration returns a record with a 0 value for subscriptions with no cost in the current month.
This will allow dashboards to accurately reflect months with zero spending, improving visibility and reporting.

*Use Case:
Teams need to monitor and report on all months, including those with zero Azure costs. Having explicit zero values ensures data completeness and supports accurate trend analysis.

Start at the Elastic Azure Billing integration entry point and inspect its existing tests for subscriptions with no current-month cost. Trace how billing records are emitted when the cost is zero, then add coverage showing that a zero-valued record is returned. Done means zero-cost subscriptions appear in the integration output without losing existing billing data.

Written by the indexing model from the issue text.