fix: handle zero length 'config.path'

[curbengh]

Handle following edge cases:

type:
  - atom
  - rss2
path: 'feed/atom.xml'
# Previously 'feed/atom.xml' was replaced by ['atom.xml', 'rss2']
# Now it should be ['feed/atom.xml', 'rss2.xml']

---

The following config previously would error out when executing

hexo-generator-feed/index.js

Line 58 in dd558c6

return feedFn.call(hexo, locals, feedType, path[type.indexOf(feedType)]);

type:
  - atom
  - rss2
path: 
  - 'feed/atom.xml'
# Previously ['feed/atom.xml'] stays the same
# Now it should be ['feed/atom.xml', 'rss2.xml']

---

type:
  - atom
  - rss2
path: []
# Previously, path stays empty
# Now it should be ['atom.xml', 'rss2.xml']

---

Noticed while testing #110

[coveralls]

Coverage remained the same at 100.0% when pulling 01db36c on curbengh:path-array into dd558c6 on hexojs:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 1ec9760 on curbengh:path-array into dd558c6 on hexojs:master.

[curbengh]

Noticed another edge case, which would cause error

type:
  - atom
path:
  - 'foo'
  - 'bar'

Edit: fixed