Fix buggy behavior with annotations/labels on preview

[lblackstone]

The unstructured GetLabels and GetAnnotations methods expect
that all of the annotations have string values, and error silently
if this is not the case. During previews that included computed
values in either the labels or annotations, the provider was
erroneously clearing these fields due to this error. This fix
manually updates those fields to avoid resetting them.

Fixes #438

[hausdorff]

I'm having trouble understanding upstream thinks this is acceptable. Can we bring this up in the appropriate upstream repositories?

[hausdorff]

Is there a way to do some sort of Go linting to fail the build if we use either of these functions? From my perspective, this is a major issue with upstream and it would be nice to make sure we don't use these APIs until they're fixed.

[lblackstone]

I don't see an obvious way to do that, but I'll keep an eye out for further problematic usage while I'm refactoring. This should only happen on preview since all the other stages will be using populated values and not promises.