Skipping else part in dataweave 2.0

You could just have the else be the preexisting variable without adding anything to it (needs initialized before this script).

payload map (value, index) -> (
  if(condition) (
    vars.variableName ++ {
    "attribute1": "value1",
    "attribute2": "value2",
    "attribute3": "value3"
    }
  )
  else vars.variableName
)

Or if vars.variableName hasn't been created yet you can default it to an empty object so you can add to it later in another script. However if the variable hasn't been initialized yet the condition in the if-else must direct the processing to the else clause. Otherwise you will get an error if trying to append an object to a variable that is null.

payload map (value, index) -> (
  if(condition) (
    vars.variableName ++ {
    "attribute1": "value1",
    "attribute2": "value2",
    "attribute3": "value3"
    }
  )
  else vars.variableName default {}
)

BTW need to use ++ to combine objects