cgroupv2: never write empty string to memory.swap.max

[lifubang]

Never write empty string to memory.swap.max
Because the empty string means set swap to 0.

For example:
with memory limit:

"memory": {
        "limit": 33554432,
        "swap": 33558528
 }

The value of memory.swap.max is 4096.
After we run runc update --memory 33554432 test, the value of memory.swap.max becomes 0.
This is a regression introduced by #2370 . I'm so sorry.

Signed-off-by: lifubang lifubang@acmcoder.com

[kolyshkin]

LGTM

[kolyshkin]

@AkihiroSuda @mrunalp PTAL

[mrunalp]

LGTM