`or` does not test possibly-undefined data values

**What steps did you take:**

I wrote this snippet into a new file `xyz.yaml`:

```yaml
#@ load("@ytt:data", "data")

#@ foo = data.values.foo or "bar"
```

**What happened:**

When running `ytt -f xyz.yaml`, the following error is generated:

```
ytt: Error:
- struct has no .foo field or method
in
template.yaml:3 | #@ foo = data.values.foo or "bar"
```

**What did you expect:**

I expected templating to complete successfully and generate an empty YAML file, per the FAQ:

> # How do I provide a default for a data value when it may not be defined? [¶](https://carvel.dev/ytt/docs/v0.38.0/faq/#how-do-i-provide-a-default-for-a-data-value-when-it-may-not-be-defined)
>
> When a value may be null, you can use or to specify a default.
>
> ```yaml
> #@ data.values.foo or "bar"
> ```

~ https://carvel.dev/ytt/docs/v0.38.0/faq/#how-do-i-provide-a-default-for-a-data-value-when-it-may-not-be-defined

**Environment:**

- ytt version (use `ytt --version`): 0.45.1
- OS (e.g. from `/etc/os-release`): macOS (via `brew install`)

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