Support empty string value for domainGuid property

[ArthurMa1978]

We previously encountered the same issue in #49612, which was fixed by #49616.
Later, the service team upgraded the spec and changed the domainGuid property to be nullable, meaning it should no longer return an empty value. As a result, in the release PR #51933, we renamed domainGuid to ActiveDirectoryDomainGuid (nullable) and added custom code to retain the original domainGuid property for backward compatibility.

However, the property can still return an empty string. Therefore, we need to update ActiveDirectoryDomainGuid to also handle empty string values, consistent with the fix implemented in #49616.