Description:
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Using filter_var() with any of the FILTER_SANITIZE_... filters will return either the sanitized value or an empty string.
This works for every single type of variable *but* an empty object.

With an empty object it returns `false` or in the case the FILTER_NULL_ON_FAILURE flag is set `false` in PHP 5 or `null` in PHP 7 (related: https://bugs.php.net/bug.php?id=67167 ).

Behaviour can be observed in numerous PHP versions here: http://phpcheatsheets.com/test/filter_extension_bool_int_float/
http://phpcheatsheets.com/test/filter_extension_strings/


Test script:
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var_dump( filter_var( null, FILTER_SANITIZE_STRING ) ); // prints ''
var_dump( filter_var( '', FILTER_SANITIZE_STRING ) ); // prints ''
var_dump( filter_var( new stdClass(), FILTER_SANITIZE_STRING ) ); // prints bool(false)

var_dump( filter_var( null, FILTER_SANITIZE_NUMBER_FLOAT ) ); // prints ''
var_dump( filter_var( '', FILTER_SANITIZE_NUMBER_FLOAT ) ); // prints ''
var_dump( filter_var( new stdClass(), FILTER_SANITIZE_NUMBER_FLOAT ) ); // prints bool(false)

var_dump( filter_var( null, FILTER_SANITIZE_STRING, FILTER_NULL_ON_FAILURE ) ); // prints ''
var_dump( filter_var( '', FILTER_SANITIZE_STRING, FILTER_NULL_ON_FAILURE ) ); // prints ''
var_dump( filter_var( new stdClass(), FILTER_SANITIZE_STRING, FILTER_NULL_ON_FAILURE ) ); // prints NULL

Expected result:
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When a sanitize filter is used on an empty object, I would expect PHP to return an empty string. This would make the behaviour more consistent.