I wanted a solution to a simple problem. Many of my tracks have blank Album Artist tag, and I wanted to replace those tags with the Artist tag whenever it is blank (equal to "null string," "empty tag," etc.) I found this action quite difficult, much more difficult than it needs to be.
First of all, what the program calls "Album Artist" is really the "Band" ID3 tag. That took me a little while to figure out in and of itself. The "Album Artist" tag, if it exists, is irrelevant. Because the tag "Album Artist" has a space in it, can't even be referred to anyways!
After a ton of searching, head-scratching, researching, and trial-and-error, I finally came up with something that works. Unfortunately, MP3tag does not seem to have a way of directly representing the null string, but I came up with this expression that works:
Action type: Format Value
Format string: $if($geql($len(%band%),1),%band%,%artist%)
The literal way to read this code is: If the length of "Band" tag is greater than or equal to one character, then the "Band" tag is equal to the "Band" tag, otherwise the "Band" tag is equal to the "Artist" tag.
The effect of this is: It will check all selected files and, if the "Band" tag (i.e. Album Artist) is blank, it will replace it with the "Artist" tag; otherwise it will leave it alone. If there is a more elegant code to solve this problem, please feel free to correct it.