Ensure that the value 'off' is properly read as a false boolean.

[wpscholar]

Currently, if a site has the default status of 'off' (disabled), the status in the sites list view shows 'enabled' as the status. This is due to checking if the non-empty string whose value is set to 'off' is falsey, which of course it is not. By running the string through filter_var() and using the FILTER_VALIDATE_BOOLEAN flag, we are able to ensure that strings are properly converted into booleans if the values are 'on' or 'off'.