DockerVersion can be unset without Jenkins restart

Setting a previously configured dockerVersion to "" resulted in invalid
URLs until Jenkins was restarted or a version was configured again.
Refactor DockerClient instantiation and improve white space handling on
docker version
Use StringUtils in more checks for empty input

as I already used isNotBlank in createDockerClient, I felt it
appropriate to adapt similar null checks to this style (at least in this
unit of code)
"Test Connection" no longer replaces the DockerClient with one based on
unsaved settings