Hide empty custom fields or specify a default value for them

Hi all,

I am using the custom_fields_block shortcode in custompress and have about a dozen custom fields. Now depending on the post some or all of these fields (text fields) may have values.
Is there a way to have the shortcode only show the custom fields that had values entered?
Or baring that is there a way to set a default value for a field if it is blank?

Many thanks,