message: Single argument string_agg works now
diff: @@ -90,6 +90,11 @@ SELECT AVG()
  statement error
  SELECT AVG(1, 2)
  +query T
  +SELECT STRING_AGG('hello')
  +----
  +hello
  +
  query TTTTT
  SELECT STRING_AGG('hello', ' '), STRING_AGG('hello', NULL), STRING_AGG(NULL, ' '), STRING_AGG(NULL, NULL), STRING_AGG('', '')
  ----
  @@ -98,9 +103,6 @@ hello NULL NULL NULL (empty)
  statement error
  SELECT STRING_AGG()
  -statement error
  -SELECT STRING_AGG('hello')
  -
  statement error
  SELECT STRING_AGG(1, 2, 3)
  