Handle nil properly in queue metrics

We support setting maxResource/ guaranteed resource to nil, but the metrics update is wrong because we don't update it from previous value update to zero/nil, for example:

https://github.com/apache/yunikorn-core/blob/7c51e82f09720b014159e0ba2f48d5e600a1f96c/pkg/scheduler/objects/queue.go#L415

https://github.com/apache/yunikorn-core/blob/7c51e82f09720b014159e0ba2f48d5e600a1f96c/pkg/scheduler/objects/queue.go#L441