SVN Error when executing svn-push script

When there's no files to be deleted according [this logic](https://github.com/xwp/wp-dev-lib/blob/master/svn-push#L119) `svn rm` commands fails in syntax as it does not get any files to remove as expected.

![screenshot from 2014-12-12 01 32 08](https://cloud.githubusercontent.com/assets/2236554/5401137/e3583fba-819e-11e4-8a51-17d48a3e0509.png)

An empty string check needs to be added.
