Remove empty quotes when category is empty.

When category = ""

Should print "category: " to file, current behaviour prints "category:
\"\"".

Latter creates an actual category, with a blank title. All posts created
since this update, without a category explicitly written in `rake`
command, or post directly edited, will belong to this category.
Simpler logic to category patch

Replace single line with cleaner, more readable Ruby.
Merge pull request #198 from parnmatt/patch-1

Remove empty quotes when category is empty.