_shortcode_parse_attrs() should always return array

Return value of _shortcode_parse_attrs() is defined as array. However in some cases function returns string instead of array (In my case it was always empty string). This can cause a problem because return value of this function is passed as first argument to "process callback" functions that are defined by hook_shortcode_info(). Such callbacks may also be defined by other modules and expect to always receive an array. Passing a string to such callback will produce "Illegal string offset" PHP warnings.