789 pav status raises exception when invoked before running tests

[hwikle-lanl]

Code review checklist:

• Code is generally sensical and well commented
• Variable/function names all telegraph their purpose and contents
• Functions/classes have useful doc strings
• Function arguments are typed
• Added/modified unit tests to cover changes.
• New features have documentation added to the docs.
• New features and backwards compatibility breaks are noted in the RELEASE.md

[hwikle-lanl]

Removed the raised exception and returned None instead, since the caller is checking for None anyways, and because it makes more sense generally.

[hwikle-lanl]

I'm also not 100% sure this unit test actually tests what it's meant to, but it passes.

[francinelapid]

Tested it on pav_tutorial. It now gives the message: User has no 'last' series for this machine.

Thanks for adding a unit test too!