Datasource XML parsing failure when missing owner or project element #1704

Describe the bug
If the datasource XML is missing the owner or project elements in the XML response, parsing fails due to an UnboundLocalError

Versions
Details of your environment, including:

• Tableau Server version (or note if using Tableau Online) Any
• Python version Any
• TSC library version v0.38

To Reproduce
Query a site that has user visibility limited. Or change the fields requested to not include any owner or project fields.

Results
UnboundLocalError in DatasourceItem.from_xml

NOTE: Be careful not to post user names, passwords, auth tokens or any other private or sensitive information.