[shim] Fix DCGMWrapperInterface nil check (#2980)

Fixes: #2979

Author: un-def

runner/internal/shim/api/handlers.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"net/http"
+	"reflect"
 
 	"github.com/dstackai/dstack/runner/internal/api"
 	"github.com/dstackai/dstack/runner/internal/log"
@@ -24,7 +25,7 @@ func (s *ShimServer) HealthcheckHandler(w http.ResponseWriter, r *http.Request)
 func (s *ShimServer) InstanceHealthHandler(w http.ResponseWriter, r *http.Request) (interface{}, error) {
 	ctx := r.Context()
 	response := InstanceHealthResponse{}
-	if s.dcgmWrapper != nil {
+	if !reflect.ValueOf(s.dcgmWrapper).IsNil() {
 		if dcgmHealth, err := s.dcgmWrapper.GetHealth(); err != nil {
 			log.Error(ctx, "failed to get health from DCGM", "err", err)
 		} else {