fix: return empty metrics data when no metric queries are built

[BSick7]

Problem

CloudWatch GetMetricData rejects a request with an empty MetricDataQueries:

400 ValidationError: The parameter MetricDataQueries is required.

For workspaces with no metrics mappings (e.g. aws/aws-fargate-task, whose base_metrics is empty), Getter.GetMetrics built zero queries and sent the request anyway, surfacing in the Nullstone UI Monitoring tab as:

unable to retrieve metrics: error retrieving metrics data: ...

Fix

After building queries, if the slice is empty, return workspace.NewMetricsData(), nil without calling CloudWatch — the UI renders an empty/no-metrics state instead of an error.

The guard is on the built queries rather than len(g.Infra.MetricsMappings) because options.Metrics acts as a filter inside BuildMetricQueries, so non-empty mappings can still produce zero queries.

The GCP getter (gcp/cloudmonitoring) iterates mappings per-series and is naturally safe when empty — no change needed there.

Testing

New TestGetter_GetMetrics_NoQueries covers both empty mappings and a filter matching nothing; both assert empty metrics data and nil error with no AWS call. go test ./aws/cloudwatch/metrics/ passes (run with -mod=mod — the vendor tree is out of sync with go.mod on master, unrelated to this change).