Fix path building in RouteModels importer

ERROR [BatchResource] Error while executing automation batch 
org.nuxeo.ecm.automation.OperationException: Failed to invoke operation FileManager.Import
	at org.nuxeo.ecm.automation.core.impl.InvokableMethod.invoke(InvokableMethod.java:140)

...
Caused by: java.lang.IllegalArgumentException: Illegal path with empty component: //default-domain
	at org.nuxeo.ecm.core.storage.sql.SessionImpl.getNodeByPath(SessionImpl.java:782)
	at org.nuxeo.ecm.core.storage.sql.ra.ConnectionImpl.getNodeByPath(ConnectionImpl.java:197)
	at org.nuxeo.ecm.core.storage.sql.coremodel.SQLSession.resolvePath(SQLSession.java:236)
	at org.nuxeo.ecm.core.api.DocumentResolver.resolveReference(DocumentResolver.java:63)
	at org.nuxeo.ecm.core.api.AbstractSession.resolveReference(AbstractSession.java:479)
	at org.nuxeo.ecm.core.api.AbstractSession.getDocument(AbstractSession.java:1174)
	at sun.reflect.GeneratedMethodAccessor125.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:616)
	at org.nuxeo.ecm.core.api.TransactionalCoreSessionWrapper.invoke(TransactionalCoreSessionWrapper.java:133)
	at $Proxy42.getDocument(Unknown Source)
	at org.nuxeo.ecm.platform.routing.core.persistence.RouteModelsZipImporter.create(RouteModelsZipImporter.java:109)
	at org.nuxeo.ecm.platform.filemanager.service.FileManagerService.createDocumentFromBlob(FileManagerService.java:249)
	at org.nuxeo.ecm.automation.core.operations.services.FileManagerImport.run(FileManagerImport.java:69)
	at org.nuxeo.ecm.automation.core.operations.services.FileManagerImport.run(FileManagerImport.java:76)