agent: Don't pass empty options to mount

With some older kernels some fs implementations don't handle empty
options strings well. This leads to failures in "setup rootfs" step.
E.g. `cgroup: cgroup2: unknown option ""`.
This is fixed by mapping empty string to `None` before passing to
`nix::mount`.

Signed-off-by: Jacek Tomasiak <jtomasiak@arista.com>
Signed-off-by: Jacek Tomasiak <jacek.tomasiak@gmail.com>

src/agent/rustjail/src/mount.rs

@@ -857,7 +857,7 @@ fn mount_from(
         dest.as_str(),
         Some(mount_typ.as_str()),
         flags,
-        Some(d.as_str()),
+        Some(d.as_str()).filter(|s| !s.is_empty()),
     )
     .inspect_err(|e| log_child!(cfd_log, "mount error: {:?}", e))?;