Unable to access nested keys when reading JSON from file

### Consul Template version

`consul-template v0.23.0 (5cf9d687)`

### `cfg1.tpl` (working)

```liquid
{{ define "some-json" }}
{
"ds": {
"meta-data": {
"local-ipv4": "127.0.0.1"
}
}
}
{{ end }}

{{- $id := executeTemplate "some-json" | parseJSON -}}

{{- index (index $id.ds "meta-data") "local-ipv4" -}}
```

`consul-template -dry -once -template=cfg1.tpl` works as expected, returning `127.0.0.1`.

### Configuration 2 (fails)

`consul-template -dry -once -template=cfg2.tpl` fails with:

```
2019/11/15 11:44:37.457560 [ERR] (cli) cfg2.tpl: execute: template: :2:10: executing "" at : error calling index: index of untyped nil
```

#### `cfg2.tpl`

```liquid
{{ $id := file "instance-data.json" | parseJSON }}
{{ index (index $id.ds "meta-data") "local-ipv4" }}
```

#### `instance-data.json`

(same as `some-json` template above)

```json
{
"ds": {
"meta-data": {
"local-ipv4": "127.0.0.1"
}
}
}
```

### Configuration 3 (fails)

`consul-template -dry -once -template=cfg3.tpl` fails with:

```
2019/11/15 11:39:57.817484 [ERR] (cli) cfg3.tpl: execute: template: :7:11: executing "" at : error calling index: index of untyped nil
```

#### `cfg3.tpl`

```liquid
{{ define "file-json" -}}
{{ file "instance-data.json" }}
{{- end }}

{{- $id := executeTemplate "file-json" | parseJSON -}}

{{- index (index $id.ds "meta-data") "local-ipv4" -}}
```

#### `instance-data.json`

As above.

### Expected behavior

Should have gotten same output regardless of source of JSON.

### Actual behavior

Ability to access JSON-parsed map structure is dependent upon source of JSON.

Start by reproducing the issue with cfg1.tpl, cfg2.tpl, and cfg3.tpl, then inspect the implementations of file, executeTemplate, and parseJSON. Compare the values passed through each path and verify that nested keys in instance-data.json produce 127.0.0.1 regardless of the JSON source.

Written by the indexing model from the issue text.