Fix warnings on invalid model indexes on startup

Various legend codepaths want to look up static metadata about a sensor.
We have this populated on startup potentially before any rows are
inserted into the model.

Calling data on an invalid index is a illegal, fortunately all of this
information is available in the header data before we have any rows.

Testing done: On the default history page I still have units and my
stacked CPU core graph correctly, but with less stderr