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

### What happened?

[Preparing the diff plan response will cause a panic](https://github.com/crossplane/upjet/blob/v1.4.1/pkg/controller/external_tfpluginfw.go#L282), 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](https://github.com/crossplane/upjet/blob/v1.4.1/pkg/controller/external_tfpluginfw.go#L282) seems to mitigate the issue:

```go
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: https://github.com/crossplane-contrib/provider-upjet-aws/issues/1357