Better error message when building DiskChopper fails

[nvaytet]

When building a DiskChopper, if a log is empty as opposed to containing just a single value, we get the error

DimensionError: Chopper field 'rotation_speed_setpoint' must be a scalar variable, got a 1d variable

The fact that it says it's 1d even though it contains nothing is confusing, because the same log contaning just 1 value gets squeezed and becomes a scalar.

The difference comes from the fact that squeezing a variable with a dimension but size 0 does nothing and keeps the dim.

sc.array(dims=['x'], values=[]).squeeze()  # --> 1d variable
sc.array(dims=['x'], values=[1]).squeeze()  # --> scalar variable

In this PR, we make the error message less confusing.