The runtime-spec just merged this PR:
https://github.com/opencontainers/runtime-spec/pull/1224
This means that it is now possible to request idmap mounts by specifying
"idmap" or "ridmap" in the mount options, without any mappings.
Let's add a check to see if they are requested in that way too.
Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>