Missing type check in ACF compatibility

The function relevanssi_index_acf() in lib/compatibility/acf.php is missing a type check for the variable $value defined on line 68. With normal ACF choice fields, this doesn’t represent a problem, but ACF fields that are added via plugin could break this. I ran into this problem when using acf-svg-icon, which returns an Array instead of a String as $value.

The condition in line 69 should add a type check for Strings (if ( is_string( $value ) ) to fix this problem. Otherwise a PHP error will be thrown when saving a post with custom ACF fields and the post cannot be saved.