[APM] Storage explorer should display sampling rate as N/A if there are no tx

Currently, the Storage Explorer displays a sampling rate of 0% if there are no transactions. This is misleading, as we can't actually infer what the sampling rate would be if there were transactions (which might not exist or might be dropped for whatever reason). We should instead display N/A. This would help the user (and us in the context of SDHs) establish more quickly that there are no transactions indexed into Elasticsearch.

https://github.com/elastic/kibana/blob/main/x-pack/plugins/apm/server/routes/storage_explorer/get_service_statistics.ts