Add a nil case to the expandWildcards function

[sergenyalcin]

Description of your changes

This PR adds a nil case to the expandWildcards function to handle the case that the value of the segment is nil.

The issue was observed while testing the GCP certificatemanager.Certificate resource. While trying to get the connection details of this resource, we try to read the self_managed[*].certificate_pem attribute of this resource. If the self_managed object is nil, then we observe an error:

cannot get connection details: cannot get connection details: cannot expand wildcards: cannot expand wildcards for segments: "self_managed[*].certificate_pem": "self_managed": unexpected wildcard usage'

If the value of this field is nil, then we do not want to return an error because there is nothing to read in the sensitive attribute. In this case, we want to handle this situation in Upjet as a specific case other than the different cases. So, by returning a specific error from the nil case, we can handle this in Upjet by ignoring this error type.

I observed a specific error type, errNotFound and used this for the nil case.

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

This was tested in the Upjet-based GCP provider. Resource: certificatemanager.Certificate

[ulucinar]

Here, we differentiate between the cases where an unexpected type is encountered versus the specified path is not found and we hit the newly added case nil here.

nit:

Suggested change

	return nil, errNotFound{errors.Errorf("%s: value of the field is nil", segments[:i])}
	return nil, errNotFound{errors.Errorf("wildcard field %q is not found in the path", segments[:i])}

[ulucinar]

Thanks @sergenyalcin, lgtm.

[turkenh]

Thanks @sergenyalcin 🙌 Returning errNotFound for that case makes sense to me.