Fixes LP: #1365144 bug.

Signed-off-by: Jorge Niedbalski R <niedbalski@gmail.com>
Merge pull request #668 from niedbalski/empty-string

cmd/set: Allow empty string fix #1348829

Fixes LP: #1348829 bug.

Since there is a plenty amount of charms accepting "" as a valid value and is a valid char, we should allow it.

Signed-off-by: Jorge Niedbalski R <niedbalski@gmail.com>