photos plugin doesn't like SITEURL being an empty string

[alfiepates]

Potentially a dupe of #1147, which was closed by the reporter immediately.

I have the following markdown file:

title: blogpost
stub: blogpost-stub
category: misc

![alt text]({photos}folder/image.jpg)

With SITEURL = '' (Pelican's default), the plugin generates the image URL photos/folder/image.jpg. This refers to http://hostname/category/blogpost-stub/photos/folder/image.jpg, instead of the correct path http://hostname/photos/folder/image.jpg.

With SITEURL = 'http://hostname' or another valid URL, the plugin generates the explicit image URL, http://hostname/photos/folder/image.jpg.

I'd rather not have to explicitly set SITEURL in my development environment since it lvies behind an reverse proxy with an ephemeral hostname.