Not doing so has a few gotchas when working with relations, particularly
with records which are not stored in the database.
I'm excluding the related content file because it's got a very peculiar
relationship with itself: the `has_one :opposite_related_content` has no
inverse; the relation itself is its inverse. It's a false positive since
the inverse condition is true:
```
content.opposite_related_content.opposite_related_content.object_id ==
content.object_id
```
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.
