only add enclosure to map if not nil

I started using Cyrogen for my blog and it is working very well for me. I appreciate that you made it.

I noticed an issue recently where the feed xml file can be invalid because of a blank . To see this behavior try the following.

Prior to the change this is the report I was receiving:

This feed does not validate.
line 42, column 0: Missing enclosure attribute: length (14 occurrences) [help]
<enclosure>

Using my post fix version here I get the following:

Let me know what you think.

Thanks

cryogen-project / cryogen-core