dockerTools: only add "/nix" if it exists

The /nix path in 4d200538 of the layer tar didn't exist for some
packages, such as cacert. This is because cacert just creates an /etc
directory and doesn't depend on any other /nix paths. If we tried
putting this directory in the tar and using overlayfs with it, we'd get
"Invalid argument" when trying to remove the directory.

We now check whether the closure is non-empty before telling tar to
store the /nix directory.

Fixes #14710.