Fix crash navigating to cluster explorer

[torchiaf]

Summary

Fixes #307

Technical notes summary

The property schema is null when navigating from embedded Epinio to cluster explorer.

  get namespaceLocation() {
    return createEpinioRoute(`c-cluster-resource-id`, {
      cluster:  this.$rootGetters['clusterId'],
      resource: this.schema.id,
      id:       this.meta.namespace,
    });
  }

Is it probably a side effect of routing which triggers a reload of Epinio pages?
Using the optional operator works fine though, namespaceLocation is not required when moving out from Epinio.