"null" data values erroneously passes schema check

[pivotaljohn]

What steps did you take:

With these files...

#@data/values-schema
---
gotta_have_this_key: true

#@data/values
---

#@ 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...

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 Schema not applied correctly in library when using library.with_data_values(). #563.
  • For this reason, this fix ought to be fixed after implementing Set the Data Values on a library a la --data-values-file #418.

Environment:

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

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[pivotaljohn]

Fixing this bug takes a behavior that was succeeding and now causes an error. This is technically a breaking change.

That said, relying on this behavior has the effect of creating a false positive. Breaks that occur from this fix will reveal potential issues that had not yet surfaced (or created enough damage to be noticed).