I've defined an mte export and read in the online help and also observed this in action that if you enclose a tag frame variable within right-angled brackets '[' and ']', it will not generate blank output if there is no value for the given tag frame variable.
However, I wanted to get some control over the 'no value' bit by using an $if formula to possibly check for an alternate source being present and fallback on a 'no value' in case the alternate contains the same value (e.g. Album Artist vs Track Artist, i.e. %albumartist% and %artist%
To my surprise the suppression of blank output then fails and outputs the line as if a value was present
used formula format (generalized for simplicity, which does exact same in practice) is
<previous export def line>[ PERFORMER "$if(%albumartist%,%artist%,)"]
the below however does work and suppresses the line completely if no value is present.
<previous export def line>[ PERFORMER "%artist%"]
I would however like to use the top example as this allows me the flexibility to not include that line if it is the same as the already exported %albumartist% line defined earlier in the export.
The bottom version only allows me to either generate a lot of lines with redundant values or skip them when no value is present.