Let Scan component handle non-scan runs gracefully

[larsfu]

Currently, if I supply a motor, which is not moved during a run, the Scan component returns an empty list of positions – I would argue that it is useful to produce a list with a single entry instead. This would allow handling scanning and non-scanning runs in the same manner in automated analyses.

[JamesWrigley]

That's reasonable, albeit a bit breaking so I'll probably wait until the next cycle to implement it.