Functions cannot return yaml scalars and return null instead

This work:

```yaml
#@ def some_data():
hello: world
#@ end

simple_key: #@ some_data()
```

This does't work

```yaml
#@ def some_data():
"hello"
#@ end

simple_key: #@ some_data()
```

neither this:

```yaml
#@ def some_data():
- hello: world
#@ end

simple_key: #@ some_data()
```

The error is not really understandable either: *Unmarshaling YAML template: yaml: line 2: did not find expected *

The issue names no files or tests. Reproduce the three YAML examples through ytt and trace how template function return values are handled; done means scalar and sequence returns are preserved instead of becoming null, with a more understandable error for invalid output.

Written by the indexing model from the issue text.