modified submenu save logic to check for existence of already queried current

The logic should be testing whether the option exists, not just whether it evaluates to boolean false, since an option may be intended to do so.

The current option value is queried on [line 119](https://github.com/nschoenholtz/wordpress-fieldmanager/blob/9078fc1b76f86265486e4854cca2819ba80194df/php/context/class-fieldmanager-context-submenu.php#L119).