using custom fields on content.php file of my theme

empty will only take variables passed by reference. so empty(some_function()) will never work. See the php docs for more information.

<?php if (get_custom_field_value("signature","")) : ?>
    &mdash;
    <?php the_category( '<span>/</span>' ); ?>      
<?php endif; ?>

You’re better off just checking for the boolean, like above. The get_custom_field_value function already takes care of making sure you get something back.

On a side note: you should prefix your function names. get_custom_field_value is very generic. yourtheme_get_custom_field_value (replacing “yourtheme” with the theme name) has less of a chance of conflicting with other function names.