bootstrap_file: do not try to create a directory with an empty name

This will happen if the chosen output file does not have a
path specified

src/blockchain_utilities/bootstrap_file.cpp

@@ -53,6 +53,8 @@ namespace
 bool BootstrapFile::open_writer(const boost::filesystem::path& file_path)
 {
   const boost::filesystem::path dir_path = file_path.parent_path();
+  if (!dir_path.empty())
+  {
     if (boost::filesystem::exists(dir_path))
     {
       if (!boost::filesystem::is_directory(dir_path))
@@ -69,6 +71,7 @@ bool BootstrapFile::open_writer(const boost::filesystem::path& file_path)
         return false;
       }
     }
+  }
 
   m_raw_data_file = new std::ofstream();