Fixes(?) volumes parameter issue

This commit adds a bit of code to check for an array containing " " -
i.e. volumes = [" "].

With docker-plugin 0.6 (Docker 1.0.0), was seeing a 500 error on POST
/containers/{id}/start

Jun 16, 2014 1:49:45 AM org.glassfish.jersey.filter.LoggingFilter log
INFO: 7 * Sending client request on thread
Computer.threadPoolForRemoting [#3]
7 > POST
http://localhost:4243/containers/debb6e2d57eb74eb3b93e4a0026cc28605660c92a7e0c873347548fae62a8863/start
7 > Content-Type: application/json
{"Binds":[""],"PortBindings":{"22/tcp":[{"HostIp":"0.0.0.0"}]},"Privileged":false,"Dns":[""]}

Jun 16, 2014 1:49:45 AM org.glassfish.jersey.filter.LoggingFilter log
INFO: 8 * Client response received on thread
Computer.threadPoolForRemoting [#3]
8 < 500
8 < Content-Length: 87
8 < Content-Type: text/plain; charset=utf-8
8 < Date: Mon, 16 Jun 2014 01:49:45 GMT
Could not create local directory '' for bind mount: mkdir : no such file
or directory!

With this change, no longer seeing 500 errors - instead:

Jun 16, 2014 1:39:17 AM org.glassfish.jersey.filter.LoggingFilter log
INFO: 7 * Sending client request on thread
Computer.threadPoolForRemoting [#3]
7 > POST
http://localhost:4243/containers/7596bb58841bc001cd1deb5335937199b3824c043430be644e988280020f4a44/start
7 > Content-Type: application/json
{"PortBindings":{"22/tcp":[{"HostIp":"0.0.0.0"}]},"Privileged":false,"Dns":[""]}

Jun 16, 2014 1:39:17 AM org.glassfish.jersey.filter.LoggingFilter log
INFO: 8 * Client response received on thread
Computer.threadPoolForRemoting [#3]
8 < 204
8 < Content-Length: 0
8 < Content-Type: text/plain; charset=utf-8
8 < Date: Mon, 16 Jun 2014 01:39:17 GMT