You want a mechanism to detect if a tag field is empty and if so, then do something related?
In Mp3tag the definition for "empty tag field" is the same as "tag field does not exist".
To check if a tag field exists respectively if a tag field has any content, you can use the Mp3tag functions $if or $if2.
$if(%ALBUM%,'yes, the tag field has any content','no, the tag field does not exist')
$if2(%ALBUM%,'no, the tag field does not exist')
DD.20100603.1955.CEST