opts: MountOpt: relax client-side validation of mount target

[thaJeztah]

The daemon already validates the target, so we don't have to validate if a target is set. Instead, we can ignore empty targets, but produce an error if a target option was set, but set to an empty value.

With this patch applied, omitting a target option is ignored by the CLI, but still invalidated by the daemon if the given mount-type requires a mount target;

docker run --rm --mount type=bind,src=/var/run/docker.sock alpine
docker: Error response from daemon: invalid mount config for type "bind": field Target must not be empty

docker run --rm --mount type=bind,src=/var/run/docker.sock,dst=../foo alpine
docker: Error response from daemon: invalid mount config for type "bind": invalid mount path: '../foo' mount path must be absolute

When passing a target option (target, dst, or destination), the CLI produces an error if the value is empty;

docker run --rm --mount type=bind,src=/var/run/docker.sock,dst= alpine
invalid argument "type=bind,src=/var/run/docker.sock,dst=" for "--mount" flag: invalid value for 'dst': mount target must be a non-empty value

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[thaJeztah]

Couple more changes remaining in a branch I was working on, but most of those are more "nice to have", and some refactoring (which I still may be continuing on). I'll open PR(s) for those later. @vvoland

[codecov-commenter]

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