eosxd: add empty object to mounts section where necessary

There is a supposed bug in Helm: https://github.com/helm/helm/issues/11617. The eosxd chart on its own generates Kubernetes manifests correctly. The problem starts when it's used as a dependency of another chart (e.g. automation/releases/cern-magnum): map entries with no values are omitted.

This commit gets around this issue by setting "null" values explicitly where there was none.