Settings page bug fix

Fixes a bug on the settings page where no post types would be displayed
under 'Show Metabox on Post Types'.

The bug occurs when a user first installs the plugin. This is because the option `address_geocoder_options` is yet to be created in the database which causes an error to be thrown when we try to loop though the results of `get_option( 'address_geocoder_options' )`.