fix: instances column of gcp_compute_instance_group errors for regional groups

[pdecat]

Without this change, the instances column of the gcp_compute_instance_group table triggers an error for regional instance groups:

# steampipe query
Welcome to Steampipe v0.24.2
For more information, type .help
> select name, instances, region_name, zone_name from gcp_compute_instance_group

Error: gcp: googleapi: Error 400: Invalid value for field 'zone': ''. Must be a match of regex '(?:[a-z](?:[-a-z0-9]{0,61}[a-z0-9])?)', invalid (SQLSTATE HV000)

+------------------------------------------------------+-----------+-------------+----------------+
| name                                                 | instances | region_name | zone_name      |
+------------------------------------------------------+-----------+-------------+----------------+
| pdecat-managed-instance-group-stateful-zonal-single  | <null>    |             | europe-west9-a |
| pdecat-managed-instance-group-stateless-zonal-single | <null>    |             | europe-west9-a |
| pdecat-unmanaged-instance-group-zonal-single         | <null>    |             | europe-west9-a |
+------------------------------------------------------+-----------+-------------+----------------+

With this change:

# steampipe query
Welcome to Steampipe v0.24.2
For more information, type .help
> select name, instances, region_name, zone_name from gcp_compute_instance_group
+--------------------------------------------------------+-----------+--------------+----------------+
| name                                                   | instances | region_name  | zone_name      |
+--------------------------------------------------------+-----------+--------------+----------------+
| pdecat-managed-instance-group-stateless-zonal-single   | <null>    |              | europe-west9-a |
| pdecat-unmanaged-instance-group-zonal-single           | <null>    |              | europe-west9-a |
| pdecat-managed-instance-group-stateful-zonal-multiple  | <null>    | europe-west9 |                |
| pdecat-managed-instance-group-stateful-zonal-single    | <null>    |              | europe-west9-a |
| pdecat-managed-instance-group-stateless-zonal-multiple | <null>    | europe-west9 |                |
+--------------------------------------------------------+-----------+--------------+----------------+

Note: instances values are null because all my test instance groups have 0 instances.

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