kubeval of "helm create test-helm" fails

Hello and apologies in advance if this is a newbie question.

I have been experiencing troubles in having kubeval validate even simple Helm charts - see https://github.com/aquariophilie/node-koa-boilerplate/pull/9 as an example.

I managed to reproduce the problem with one of the easiest test I could think about

```bash
helm version
helm create test-helm

docker run -it \
-v `pwd`/test-helm:/test-helm \
garethr/kubeval test-helm/*
```

Here is the result:

```text
gmacario@gmpowerhorse:~ $ helm version
version.BuildInfo{Version:"v3.7.0", GitCommit:"eeac83883cb4014fe60267ec6373570374ce770b", GitTreeState:"clean", GoVersion:"go1.16.8"}
gmacario@gmpowerhorse:~ $ helm create test-helm
Creating test-helm
gmacario@gmpowerhorse:~ $ docker run -it \
> -v `pwd`/test-helm:/test-helm \
> garethr/kubeval test-helm/*
ERR - Could not open file test-helm/charts
ERR - test-helm/Chart.yaml: Missing 'metadata' key
ERR - Could not open file test-helm/templates
ERR - test-helm/values.yaml: Missing 'metadata' key
gmacario@gmpowerhorse:~ $
```

You can find more details at https://github.com/aquariophilie/node-koa-boilerplate/issues/7 and https://github.com/aquariophilie/node-koa-boilerplate/pull/10

Thanks for your attention -- Gianpaolo