Datasource XML parsing failure when missing owner or project element

[jorwoods]

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.