Fix for empty ("") appbase config default value

[heifner]

Change Description

• Specifying an empty "" default value would fail when trying to create a new config.ini file.
  • Example:

void new_plugin::set_program_options(options_description& cli, options_description& cfg) {
   auto op = cfg.add_options();
   op("option-one", bpo::value<std::string>()->default_value(""), "Option");
}

• See Fix for empty ("") appbase config default value appbase#81 & Fix for empty ("") appbase config default value - 2.0 appbase#82

Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions