types: Prevent panics when ValueFromTerraform received nil values

[bluekeyes]

When getting an object under a missing (null) attribute, GetAttribute would panic instead of returning null. Also add a unit test for this case.

Fixes #207.

[bflad]

Hi @bluekeyes 👋 Thank you for submitting this. Fixing the panic certainly seems ideal in this situation.

In terms of implementation, the preference should likely be the following for the type implementations:

• Check for nil first, return null type in that situation
• Perform the type check; types should ensure they align correctly upfront
• Perform the null/unknown checks

This class of panic fix should be performed in all framework defined types in the types package and a quick nil input unit test should be included for each type's ValueFromTerraform such as TestObjectTypeValueFromTerraform. The TestStateGetAttribute unit testing addition should likely also get replicated across Config and Plan to try and prevent future regressions should those implementations diverge for whatever reason.

Please let us know if you have time to make these changes and thanks again for reporting this and contributing! 😄

[bflad]

Oh! Also feel free to create a CHANGELOG entry as well for this by creating a .changelog/208.txt file with contents such as:

```release-note:bug
types: Prevented panics when `ValueFromTerraform` received `nil` values
```

[bluekeyes]

Thanks for the review - I think I've addressed your main concerns. I did not add the nil type check to any of the primitive types, as they don't do any explicit type checks right now, deferring to tftypes.Value.As instead, but I could go through and do this if you'd like. It felt a bit weird without any other existing type checks.

[bflad]

Thank you again, @bluekeyes, looks good to me 🚀

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