Filter empty lines from "docker ls" response (#52081) (#52373)

* Filter empty lines from docker ls response

In order to cut down on test time, our docker/vagrant tests build the
docker image outside of the vagrant VM. When we get around to launching
the Vagrant VM, we mount that already-built docker image to a known
location. At that point, we need to load the docker image. But we only
want to load it once. As we're running tests, we use "docker ls" to
check whether the local image is loaded for use. Empty output from the
particular ls invocation means no image is loaded.

There was a bug in how we checked this. In Java, splitting an empty
string will yield an array containing one empty string. So when we're
counting the output from the docker ls command, we need to filter out
empty lines in order to proceed to loading the image for docker tests.