Checks environment variables for empty strings

If an environment variable is an empty string, cadvisor panics and fails to start

container/docker/handler.go

@@ -252,12 +252,14 @@ func newDockerContainerHandler(
 	// split env vars to get metadata map.
 	for _, exposedEnv := range metadataEnvs {
 		for _, envVar := range ctnr.Config.Env {
+			if envVar != "" {
 				splits := strings.SplitN(envVar, "=", 2)
 				if splits[0] == exposedEnv {
 					handler.envs[strings.ToLower(exposedEnv)] = splits[1]
 				}
 			}
 		}
+	}
 
 	return handler, nil
 }