JCR import command fails when replacing existing nodes

Steps to reproduce

In demo author:

1. Export an asset from dam app
2. Open importer subapp of Jcr-Tools app
3. Select dam repository, root path and behaviour: replace existing nodes with the same ID
4. Press execute

Expected results

Node is imported correctly and replaced

Actual results

There is an error message: 

javax.jcr.query.InvalidQueryException: Query:
SELECT * FROM [nt:resource] WHERE(*); expected: NOT, (

Development notes

The query is not correctly generated in method info.magnolia.importexport.command.JcrImportCommand.validate(Node, Context, NodeIterator). The value of statement is: 

SELECT * FROM [nt:resource] WHERE 

It's assumed that there's always a where clause while there are some cases in which it's not the case. Adding an empty string check in line 118 will fix this issue.