Accept objects with DynamicPseudoType with null values

[kjagiello]

The value of a DynamicPseudoType should be allowed to be either unknown or null. The check for the latter was missing and this PR adds it. Let me know if there are any more tests needed.

Solves #115

[hashicorp-cla]

All committers have signed the CLA.

[paddycarver]

Hey @kjagiello! Thanks for the PR. :) Any chance you wouldn't mind adding a changelog entry for this? You can do that by including something like

```release-note:bug
Fixed a panic when using DynamicPseudoType attributes in an object.
```

in .changelog/116.txt.

Thanks!

[kjagiello]

Of course! Added it now.

[paddycarver]

Thank you!

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