Preventing empty output lines in export

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.

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