Can't add items for NetworkPolicy

Reproduction:

```yaml
# config.yaml
networkPolicy:
enabled: true
ingress:
grpc:
- podSelector:
matchLabels:
foo: bar
```

```
helm lint --debug -n cockroachdb ./cockroachdb -f ./config.yaml
==> Linting ./cockroachdb
[ERROR] templates/: template: cockroachdb/templates/_helpers.tpl:14:14: executing "cockroachdb.fullname" at <.Values.fullnameOverride>: can't evaluate field Values in type []interface {}

Error: 1 chart(s) linted, 1 chart(s) failed
helm.go:75: [debug] 1 chart(s) linted, 1 chart(s) failed
main.newLintCmd.func1
/private/tmp/helm-20200213-73045-zsskjg/src/helm.sh/helm/cmd/helm/lint.go:113
github.com/spf13/cobra.(*Command).execute
/private/tmp/helm-20200213-73045-zsskjg/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:826
github.com/spf13/cobra.(*Command).ExecuteC
/private/tmp/helm-20200213-73045-zsskjg/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:914
github.com/spf13/cobra.(*Command).Execute
/private/tmp/helm-20200213-73045-zsskjg/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:864
main.main
/private/tmp/helm-20200213-73045-zsskjg/src/helm.sh/helm/cmd/helm/helm.go:74
runtime.main
/usr/local/Cellar/go/1.13.8/libexec/src/runtime/proc.go:203
runtime.goexit
/usr/local/Cellar/go/1.13.8/libexec/src/runtime/asm_amd64.s:135
```

Jira issue: HELM-42