show: use getter result to determine "current" presence (voltage range)

Improve the condition which presents the "(current)" decoration when
voltage ranges are listed in --show output. Use an explicit boolean
which is derived from the config getter's return code. Avoid "forcing"
a comparison against a zero value, which can be a valid item in the list
of supported voltages.

This is similar to commit 0171a4a7a49f. See more context (-U10) for the
motivation of the change. Was observed with the kingst-la2016 driver.