Tcv get ids nbi update fix for empty field

There are some nbi ids which are being filled with data but no time array. This is causing issues in user code since the data array should not be longer than the time array. Example demo:

shot=[68340];
idss_nbi = gdat(shot,'ids','source','nbi');

idss_nbi.nbi.unit{2}.power_launched
idss_nbi.nbi.unit{2}.energy
idss_nbi.nbi.unit{2}.beam_current_fraction
idss_nbi.nbi.unit{2}.beam_power_fraction

See related Redmine issue