You mean not showing the line with the custom field (as shown on screenshot) in view.php at all when it's not set ?

If so, this is currently not feasible, and TBH I'm not sure this is desirable, as it would make the user interface unpredictable.

Anyway, you could achieve this by modifying (at your own risk) the foreach loop in bug_view_inc.php line 641, skipping the loop if the custom field value is not set, like this (not tested):

 # Custom Fields
 if( isset( $t_issue['custom_fields'] ) ) {
        foreach( $t_issue['custom_fields'] as $t_custom_field ) {
+               if( custom_field_get_value( $t_custom_field['field']['id'], $f_bug_id ) === null ) {
+                       continue;
+               }
                $t_def = custom_field_get_definition( $t_custom_field['field']['id'] );
                $t_class = $t_def['type'] == CUSTOM_FIELD_TYPE_TEXTAREA ? ' cfdef-textarea' : '';

image.png (32,661 bytes)   

image.png (32,661 bytes)