Empty value for a block or a map in forProvider causes a panic

[villevsv-upcloud]

What happened?

Preparing the diff plan response will cause a panic, if the diff.Value1 for a path is nil.

How can we reproduce it?

1. Apply a resource with non-empty values for block or map fields.
2. Updating the resource with empty values such as [] and {} will cause a panic.

This is most likely related to the way the diff in terraform-plugin-go seems to work, but also how the library handles empty values when marshaling to msgpack.

Using a forked version of upjet repo with a small change made to the external_tfpluginfw.go:282 seems to mitigate the issue:

		if diff.Value1 != nil && diff.Value1.IsKnown() && !diff.Value1.IsNull() {

In addition, deleting the resource with also cause some troubles, if the field with the empty value is marked as required.

Related omitempty issue: crossplane-contrib/provider-upjet-aws#1357