importer: do not trigger speedupFileStorageTxnLookup uselessly

When importing a FileStorage DB without interruption and without having to
serve client nodes, the index built by speedupFileStorageTxnLookup is useless.
Such case happens when doing simulation tests and on DB with many oids,
it can take a lot of time and memory for nothing.

neo/storage/database/importer.py

@@ -356,7 +356,7 @@ class ImporterDatabaseManager(DatabaseManager):
     def _import(self):
         p64 = util.p64
         u64 = util.u64
-        tid = p64(self.zodb_tid + 1)
+        tid = p64(self.zodb_tid + 1) if self.zodb_tid else None
         zodb_list = []
         for zodb in self.zodb:
             try: