lib: Fix cockpit-po-plugin instantiation without options
lib: Fix cockpit-po-plugin instantiation without options

starter-kit and other projects that just have one webpack have no need
to specify any option. The plugin previously crashed with

    TypeError: Cannot read property 'subdir' of undefined

 in that case. Default to an empty object if options are undefined.

cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,garrett/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,garrett/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,garrett/cockpit,martinpitt/cockpit,martinpitt/cockpit,garrett/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit