update:fix update pids-limit=0 error

[ningmingxiao]

when try nerdctl -n k8s.io update --cpuset-cpus 0-2 ffe
this code

nerdctl/cmd/nerdctl/container/container_update.go

Lines 327 to 329 in c41b394

	if spec.Linux.Resources.Pids == nil {
	spec.Linux.Resources.Pids = &runtimespec.LinuxPids{}
	}

will make spec.Linux.Resources.Pids =0 it will not work on rust-shim(will cause rshim set pidlimit=0 goshim works well.).
https://github.com/containerd/rust-extensions
will cause

./nerdctl  -n k8s.io  exec  ffe  date
FATA[0000] Others("Other: OCI runtime exec failed: exec failed: unable to start container process: read init-p: connection reset by peer"): unknown

runc log 
runtime/cgo: pthread_create failed: Resource temporarily unavailable

SIGABRT: abort

PC=0x7f128ec969cf m=0 sigcode=18446744073709551610

[ningmingxiao]

fix: #3784

[ningmingxiao]

@AkihiroSuda can you review this pr ,thank you?