Fix CustomSourceTime with times completely outside envelope range

[caseyflex]

Fixes this issue:
#1899

The behavior of CustomSourceTime is to interpolate for times within the envelope definition time range, and use nearest selection for times outside the envelope definition time range. When evaluating at a list of times which is completely outside the envelope definition time range, xarray.interp fails, because we pass it an empty list. This PR checks separately for this case, when no interpolation is needed.

[momchil-flex]

Thanks, the fix looks good, but don't we still want a validator to error or at least warn that (as far as I understand?) this source will basically inject nothing in the simulation?

[caseyflex]

Thanks, the fix looks good, but don't we still want a validator to error or at least warn that (as far as I understand?) this source will basically inject nothing in the simulation?

It will actually inject something -- the usual behavior of CustomSourceTime is to just use the first or last value of the envelope provided for times outside of the envelope definition time range. But yeah, we should probably validate this because this is unlikely to be intentional.