added presence check to index and show to make empty fields not render.

Fixes #81
```ruby
def should_render_show_field? document, solr_field
    should_render_field?(solr_field, document) && document_has_value?(document, solr_field)
end
```

```ruby
def should_render_field? field_config, *args
    return field_config if field_config === true or field_config === false
    
    if_value = !field_config.respond_to?(:if) || field_config.if.nil? || evaluate_configuration_conditional(field_config.if, field_config, *args)
    
    unless_value = !field_config.respond_to?(:unless) ||  field_config.unless.nil? || !evaluate_configuration_conditional(field_config.unless, field_config, *args)

    if_value && unless_value
  end
```