Fix(?) for DockerTemplate volumes param

Not 100% sure I know enough about the code to say for sure, but I *think* I encountered an issue with the latest (0.6) release of the plugin regarding the "volumes" parameter that was introduced a few commits ago. Looks like the parsing of the "volumeString" parameter was somehow producing and array containing an empty string [" "], which the Docker API didn't like (see below). This code change seems to have fixe the issue that I was seeing tho' I wasn't able to verify whether the "volumes" feature was still working.

With docker-plugin-0.6 (Docker 1.0.0), seeing 500 on `POST` to `/containers/{id}/start`:

		  Jun 16, 2014 1:53:59 AM org.glassfish.jersey.filter.LoggingFilter log
		  INFO: 11 * Sending client request on thread Computer.threadPoolForRemoting [#3]
		  11 > POST http://localhost:4243/containers/065824b714bb1865d4a293867b66d29aff164b93ff877a5b108199b0c0388014/start
		  11 > Content-Type: application/json
		  {"Binds":[""],"PortBindings":{"22/tcp":[{"HostIp":"0.0.0.0"}]},"Privileged":false,"Dns":[""]}

		  Jun 16, 2014 1:53:59 AM org.glassfish.jersey.filter.LoggingFilter log
		  INFO: 12 * Client response received on thread Computer.threadPoolForRemoting [#3]
		  12 < 500
		  12 < Content-Length: 87
		  12 < Content-Type: text/plain; charset=utf-8
		  12 < Date: Mon, 16 Jun 2014 01:53:59 GMT
		  Could not create local directory '' for bind mount: mkdir : no such file or directory!

After this fix, getting 204 as expected:

		  Jun 16, 2014 2:41:29 AM org.glassfish.jersey.filter.LoggingFilter log
		  INFO: 3 * Sending client request on thread Computer.threadPoolForRemoting [#3]
		  3 > POST http://localhost:4243/containers/9158b1c4d44cebc8c86deb04952cd66d852b27428aa28878c92a5b03804b1f60/start
		  3 > Content-Type: application/json
		  {"PortBindings":{"22/tcp":[{"HostIp":"0.0.0.0"}]},"Privileged":false,"Dns":[""]}

		  Jun 16, 2014 2:41:30 AM org.glassfish.jersey.filter.LoggingFilter log
		  INFO: 4 * Client response received on thread Computer.threadPoolForRemoting [#3]
		  4 < 204
		  4 < Content-Length: 0
		  4 < Content-Type: text/plain; charset=utf-8
		  4 < Date: Mon, 16 Jun 2014 02:41:30 GMT