Fix warning when computing history on empty document model

Pre-requisite:
Set Nuxeo not to use Elasticsearch for the audit in nuxeo.conf

audit.elasticsearch.enabled=false

Steps to reproduce:

1. Log in
2. Navigate to the personal workspace
3. Push the + button and choose Folder
4. At this step, it will produce this message in the logs

   2021-02-24T11:50:12,447 WARN  [DocumentHistoryPageProvider] No core session found: cannot compute all info to get complete audit entries
   2021-02-24T11:50:12,447 WARN  [DocumentHistoryPageProvider] No core session found: cannot compute all info to get complete audit entries
   

It corresponds to the call to /nuxeo/api/v1/path/default-domain/workspaces/espace/@emptyWithDefault?type=Folder with all enrichers

enrichers-document: hasContent,firstAccessibleAncestor,permissions,breadcrumb,preview,favorites,subscribedNotifications,thumbnail,renditions,pendingTasks,runnableWorkflows,runningWorkflows,collections,audit,subtypes,tags,publications

Among these enrichers, "audit" will try to compute the history of the empty document model and because of that, these lines DocumentHistoryPageProvider will try to read the coreSession from the empty document model and it will return null => this case is not taken into account by the current code

https://github.com/nuxeo/nuxeo/blob/release-10.10-HF41/nuxeo-features/nuxeo-platform-audit/nuxeo-platform-audit-api/src/main/java/org/nuxeo/ecm/platform/audit/api/document/DocumentHistoryPageProvider.java#L92-L95

Expected behavior: no warn is produced in the logs

NB: we could consider the problem comes from WebUI which makes this request with all enrichers, and most of them are probably useless for an empty document model ...