[resource_datadog_logs_index] Fix having zero retention logs indexes with flex

[Prepager]

Motivation

The resource currently has two issues:

• On creation: You can not create an index with only flex retention and no standard retention defined.
• On update: You can not remove the standard retention to have only the flex retention.

Changes

Context: While the endpoint is marked as a PUT it acts as a PATCH (docs)

On creation
If the resource has a flex_retention_days set, but no retention_days, it will default to 0 for retention_days.

On update
The previous GetOk check would turn 0 into null, making it impossible to remove the retention_days.
The new condition does an explicit IsNull check to allow through 0 values.

[arc013]

I'm doing a blind approval to not block you but I'm a little unsure why this change requires logs-forwarding's approval as it seems to be about index (which logs-forwarding team has close to zero context on).