About the script error: there is a missing $ in front of the if.
And yes, if a value is empty $lt would consider it smaller. But maybe combine it with an $if2, which takes the first non empty value:
$set(date,$if2($if($lt(%_recording_firstreleasedate%,%originaldate%),%_recording_firstreleasedate%,%originaldate%),%_recording_firstreleasedate%,%originaldate%,%date%))
I hope I got this right, did not test it and wrote it on my mobile 
But this is supposed to use your condition of the smaller value, but if that turns out to be empty falls back to the first non-empty value.