Ensure fields in bunny-go struct message are set to nil instead of the types default value

[fho]

When converting fields from a resource to the bunny-go struct, schema.ResourceData.Get() is called via the type get*Ptr helper functions.
schema.ResourceData.Get() returns the types default value if a field is not set.
Investigate if in the bunny-go message fields are set to their default value instead of nil when they are unset in the provider.
If this is the case, evaluate using GetOk instead.

see also the commit message: 5-stones@773b9be