For empty collection name, the NamespaceString constructor creates ns string with an dot character at the end of the string, for example, `NamespaceString("test", "")`, the `_ns` will be "test.".

We should keep the same rule for NamespaceStringUtil when deserialize NamespaceString object with db name and empty collection name (see example code). Otherwise, we have the risk to break some downstream systems.