Docker compat: Return null when swappiness is -1

[inknos]

Fixes: #23824

Does this PR introduce a user-facing change?

`podman inspect` returns MemorySwappiness: null when it's not set

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[inknos]

I speak a very broken Go, and I am not sure whether this is the right place to fix it, so I need some expertise here. @baude @Luap99

(It makes some sense in my head though)

[inknos]

@Luap99 makes more sense now.

However, I am checking it on my system and MemorySwappiness is 0...

$ ./bin/podman inspect practical_germain | grep Swap
               "MemorySwap": 0,
               "MemorySwappiness": 0,

Not sure if I am missing something or if this is part of a bigger problem. Shouldn't it be -1 on systems with cgroupsv2?

$ grep cgroup /proc/filesystems
nodev   cgroup
nodev   cgroup2