"null" data values erroneously passes schema check

**What steps did you take:**

With these files...
```yaml
#@data/values-schema
---
gotta_have_this_key: true
```

```yaml
#@data/values
---
```

```yaml
#@ load("@ytt:data", "data")
---
data.values: #@ data.values
```
.. ran this ...
```
$ ytt -f .
```

**What happened:**
```
data.values: null
```

**What did you expect:**
I expected to see a schema error message, like...

```console
ytt: Error: Overlaying data values (in following order: values.yml):
One or more data values were invalid
====================================

values.yml:
|
2 | ---
|

= found: null
= expected: map (by schema.yml:2)
```

**Anything else you would like to add:**

- Fixing this bug would be a breaking change: it's turns what is currently a "successful" result and error outcome.
- This behavior is being used as part of a workaround for situations like #563.
- For this reason, this fix ought to be fixed _after_ implementing #418.

**Environment:**

- ytt version (use `ytt --version`): v0.38.0

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Reproduce the case with the schema, data-values, and template snippets by running `ytt -f .`, then trace the data-values schema check that accepts the null result. Compare the observed output with the expected schema error and account for the dependency on #418 and the workaround described for #563. Done means invalid null data values produce the documented schema error without breaking the intended follow-up behavior.

Written by the indexing model from the issue text.