If an empty result set is passed to exportCSV, don't return all docs.

This now matched the behaviour of exportXLS.

Products/CMFPlomino/PlominoView.py

@@ -581,7 +581,7 @@ def exportCSV(self,
                 logging.exception('Bad quoting: %s'%quoting, exc_info=True)
                 quoting = csv.QUOTE_NONNUMERIC
 
-        if not brain_docs:
+        if brain_docs is None:
             brain_docs = self.getAllDocuments(getObject=False)
 
         columns = [c.id for c in self.getColumns()]