Plugin Options Array Set to Undefined

WordPress provides a default method, wp_parse_args, to combine a set of default options and a user-defined set of options. This method uses PHP’s native array_merge for arrays, but also works when either of the two option sets is an object or a WordPress options string.

You simply provide an array of default options and an array of set options, and WordPress fills the options that are missing in the options set by their default value.

You can find this code, adjusted for your example, below:

$options = get_option( 'plugin_options' );
$options_default = array(
    'plugin_option_1' => '',
    'plugin_option_2' => '',
    'plugin_option_3' => '',
    'plugin_option_4' => '',
    'plugin_option_5' => ''
);

$options = wp_parse_args( $options, $options_default );