/Sync doesn't populate nodes in elasticsearch

## Gentics Mesh Version, operating system, or hardware.
- v1.7.6
- v1.7.9

## Operating System
- K8s

## Elasticsearch 7.7.0

## Problem
**/search/sync** doesn't work properly.

## Reproducer
1.-Create a new export file using the API **/admin/graphdb/export**
2.- Import using **/admin/graphdb/import**
3.- Clear indexes **/search/clear**
4.- Sync indexes using **/search/sync**

## Expected behaviour
After restoring a backup or importing an export file and clear (**/search/clear**) and sync (**/search/sync**), the indexes in elasticseach should be populated as before.

## Actual behaviour
After restoring a backup or importing an export file and clear (**/search/clear**) and sync (**/search/sync**), the indexes in elasticseach are **empty**