Change the way we retrireve the images urls

For some reason the paperclip method `attachment.exists?' was
returning nil even when `attachment.url(style)' was correctly
returning the url/path of the attachment.

Therefore returning `nil' was causing to raise an error in the
method `image_tag'.

With this change we make sure we return the image url it it's
available, or an empty string if it's not, but never a null value.

app/models/concerns/galleryable.rb

@@ -6,7 +6,7 @@ module Galleryable
     accepts_nested_attributes_for :images, allow_destroy: true, update_only: true
 
     def image_url(style)
-      image.attachment.url(style) if image && image.attachment.exists?
+      image&.attachment&.url(style) || ""
     end
   end
 end

app/models/concerns/imageable.rb

@@ -6,7 +6,7 @@ module Imageable
     accepts_nested_attributes_for :image, allow_destroy: true, update_only: true
 
     def image_url(style)
-      image.attachment.url(style) if image && image.attachment.exists?
+      image&.attachment&.url(style) || ""
     end
   end
 end