Prevent empty enclosure tags from being added

Currently ffpc will include empty enclosure tags if you include a node without a file attached. This can cause problems when using something like the RSS Remote Enclosure module, which also creates enclosure tags.

For example, I have a list of "Podcast" nodes, that should either reference a local file (using ffpc) or an external file (using encl_remote or some other module). If I create a feed of podcast nodes, I'll end up with a list of nodes, some of which have one enclosure (generated by ffpc pointing at a local file) and some nodes that have two enclosures (one generated something like encl_remote pointing to a remote file, and an empty enclosure tag generated by ffpc).

I think ffpc should just not include an enclosure tag if it doesn't have a file to include, which will allow for the module to integrate better with other RSS modules. I've attached a patch against 1.0 that does this. Hope it helps and please let me know if there are any changes I can make to it that'd be helpful.