Pass measurements recorded without limits

A measurement that records a value but has no validators now passes, as in OpenHTF and TestStand, instead of showing Unset.

• Fixed the outcome of a measurement with a value and no validators: it is PASS again in the Framework, the station kiosk and the Robot Framework connector. A measurement with no value and no validators stays Unset.
• Fixed TestStand and WATS imports: a step reported as Done now imports as PASS, as ATML imports already did.
• Updated the API reference and the Python, C#, Rust and C++ clients to describe the new rule.