PS-11002 fix: changing storage prefix fails

https://perconadev.atlassian.net/browse/PS-11002

Having just single 'metadata.json' in the storage is no longer considered an
integrity violation for the 'storage' initialization logic. In this case we simply
validate this storage metadata file and return early (basically, consider this layout
an empty storage).

Author: percona-ysorokin

src/binsrv/storage.cpp

@@ -83,6 +83,13 @@ storage::storage(const storage_config &config,
   load_metadata();
   validate_metadata(replication_mode);
 
+  // if after metadata erasure 'storage_objects' is empty, then this mean
+  // that it has only metadata in it that passes validation and we can
+  // consider it as an initialized empty storage, so just return
+  if (storage_objects.empty()) {
+    return;
+  }
+
   const auto binlog_index_it{storage_objects.find(default_binlog_index_name)};
   if (binlog_index_it == std::cend(storage_objects)) {
     util::exception_location().raise<std::logic_error>(