[Bug]: Shim fails on `/api/instance/health` if `libdcgm.so` is not installed

[un-def]

Steps to reproduce

1. Make sure libdcgm.so.4 is not installed
2. Run curl http://localhost:10998/api/instance/health on the instance
3. Check /root/.dstack/shim.log

Actual behaviour

Null pointer dereference:

[ 68.516645] cloud-init[1852]: created by net/http.(*Server).Serve in goroutine 1
13 Aug 11:42:07.000
[ 68.516534] cloud-init[1852]: /opt/hostedtoolcache/go/1.23.8/x64/src/net/http/server.go:2092 +0x5d0
13 Aug 11:42:07.872
[ 68.516421] cloud-init[1852]: net/http.(*conn).serve(0xc00031b0e0, {0xb19828, 0xc0000bd7d0})
13 Aug 11:42:07.992
[ 68.516307] cloud-init[1852]: /opt/hostedtoolcache/go/1.23.8/x64/src/net/http/server.go:3210 +0x8e
13 Aug 11:42:07.976
[ 68.516196] cloud-init[1852]: net/http.serverHandler.ServeHTTP({0xc00012ea80?}, {0xb18858?, 0xc000276460?}, 0x6?)
13 Aug 11:42:07.040
[ 68.515966] cloud-init[1852]: /opt/hostedtoolcache/go/1.23.8/x64/src/net/http/server.go:2747 +0x1ca
13 Aug 11:42:07.936
[ 68.515855] cloud-init[1852]: net/http.(*ServeMux).ServeHTTP(0xc0003d9b88?, {0xb18858, 0xc000276460}, 0xc00038cb40)
13 Aug 11:42:07.080
[ 68.515741] cloud-init[1852]: /opt/hostedtoolcache/go/1.23.8/x64/src/net/http/server.go:2220 +0x29
13 Aug 11:42:07.032
[ 68.515626] cloud-init[1852]: net/http.HandlerFunc.ServeHTTP(0xc0002760e0?, {0xb18858?, 0xc000276460?}, 0xc0003d9b38?)
13 Aug 11:42:07.928
[ 68.515514] cloud-init[1852]: /home/runner/work/dstack/dstack/runner/internal/api/common.go:111 +0x77
13 Aug 11:42:07.968
[ 68.515399] cloud-init[1852]: github.com/dstackai/dstack/runner/internal/api.(*Router).AddHandler.JSONResponseHandler.func1({0xb18858, 0xc000276460}, 0xc00038cb40)
13 Aug 11:42:07.984
[ 68.515293] cloud-init[1852]: /home/runner/work/dstack/dstack/runner/internal/shim/api/handlers.go:28 +0x95
13 Aug 11:42:07.096
[ 68.515177] cloud-init[1852]: github.com/dstackai/dstack/runner/internal/shim/api.(*ShimServer).InstanceHealthHandler(0xc000328180, {0x8?, 0x9848e0?}, 0xc000122501?)
13 Aug 11:42:07.112
[ 68.515061] cloud-init[1852]: /home/runner/work/dstack/dstack/runner/internal/shim/dcgm/wrapper_linux.go:77 +0x1d
13 Aug 11:42:07.040
[ 68.514948] cloud-init[1852]: github.com/dstackai/dstack/runner/internal/shim/dcgm.(*DCGMWrapper).GetHealth(0x14?)
13 Aug 11:42:07.088
[ 68.514846] cloud-init[1852]: /opt/hostedtoolcache/go/1.23.8/x64/src/runtime/panic.go:791 +0x132
13 Aug 11:42:07.048
[ 68.514730] cloud-init[1852]: panic({0x98fda0?, 0xe73240?})
13 Aug 11:42:07.888
[ 68.514589] cloud-init[1852]: /opt/hostedtoolcache/go/1.23.8/x64/src/net/http/server.go:1947 +0xbe
13 Aug 11:42:07.016
[ 68.514474] cloud-init[1852]: net/http.(*conn).serve.func1()
13 Aug 11:42:07.896
[ 68.514355] cloud-init[1852]: goroutine 27 [running]:
13 Aug 11:42:07.080
[ 68.514263] cloud-init[1852]: 2025/08/13 09:42:07 http: panic serving 127.0.0.1:52508: runtime error: invalid memory address or nil pointer dereference

Expected behaviour

No response

dstack version

0.19.23

Server logs

Additional information

No response