Fix Undertow Deployment Metrics

Undertow deployment metrics are relevant only when /subsystem=undertow's statistics-enabled is true.

When that it not the case (and metrics can not be computed) the metrics' read handler should not return a meaningless value (e.g. 0) as it already specifies an undefined metric value in the metrics' attribute definition.

That change and WFCORE-4190 will prevent exposing Undertow metrics when they do not report actual metric from Undertow underlying runtime.