Fix NPE on propagateRestoreFromColdStorage

propagateRestoreFromColdStorage: Skip record after failure: propagateRestoreFromColdStorage-01:+19

Action: propagateRestoreFromColdStorage fails on record: propagateRestoreFromColdStorage-01:+19 after retries.

java.lang.NullPointerException: null
	at org.nuxeo.coldstorage.service.ColdStorageServiceImpl.getContentBlobKey(ColdStorageServiceImpl.java:570) ~[nuxeo-coldstorage-10.0.0.jar:?]
	at org.nuxeo.coldstorage.service.ColdStorageServiceImpl.proceedRestoreMainContent(ColdStorageServiceImpl.java:429) ~[nuxeo-coldstorage-10.0.0.jar:?]
	at org.nuxeo.coldstorage.action.PropagateRestoreFromColdStorageContentAction$PropagateRestoreFromColdStorageContentComputation.compute(PropagateRestoreFromColdStorageContentAction.java:84) ~[nuxeo-coldstorage-10.0.0.jar:?]
	at org.nuxeo.ecm.core.bulk.action.computation.AbstractBulkComputation.lambda$processBatchOfDocuments$1(AbstractBulkComputation.java:147) ~[nuxeo-core-bulk-10.10-HF71.jar:?]