Fixes to handle no published posts

[jlVidal]

The changes avoid unexpected exceptions on Hexo when there's no posts on the blog, or just drafts.

[Xuanwo]

check failed with Template render error: (unknown path) [Line 7, Column 35]

[jlVidal]

I don't know why is not compiling a ternary if, in my computer works fine. Any ideas?

[Xuanwo]

It looks like you return a empty string, maybe this cause a problem ? https://github.com/hexojs/hexo-generator-feed/commit/d0a75316d4df1136b74f4bd3e316b99d40fc7bef#diff-2d50619f126a12162223a89f7b15c54eL7

[michaelowens]

Still an issue to date

[tomap]

LGTM