Using nil for wordpress default socket

This PR is to fix issue #161 for jekyll-import. The default socket of an empty string causes issues while passing nil as a default does not. 

I could not come up with an automated test for this as we'd need a test db to use Sequel with. You can recreate this issue on command line with an error occuring pre-PR with:
```bash
ruby -rubygems -e 'require "jekyll-import";
  JekyllImport::Importers::WordPress.run({
  "dbname" => "your_db",
  "user" => "root"
})'
```

Using the change in the PR the above will work.