Preventing empty output lines in export

[DetlevD]

@just_Addict

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. ...
To my surprise the suppression of blank output then fails and outputs the line as if a value was present ...

Yes, in Mp3tag scripting language the [...] square bracket clamp is an operator.
Once there is any tagfield content operator within the square bracket clamp, e. g. %TITLE%, which owns a value, then the entire content of the square bracket clamp will be come into usage.

This scripting line ...

[PERFORMER "$if(%ALBUMARTIST%,%ARTIST%,)"]

... or written as ...

['PERFORMER "'$if(%ALBUMARTIST%,%ARTIST%,)'"']

... has the meaning of ...
"If ALBUMARTIST exists, then write ARTIST, else write nothing."

If ALBUMARTIST does not exist, but ARTIST exist, then the result is the text:

'PERFORMER ""'

Probably you want this expression ...

['PERFORMER "'$if2(%ALBUMARTIST%,%ARTIST%)'"']

... with the meaning of ...
"If both tagfields do not exist, then the result is an empty string."

DD.20160314.1042.CET