Another brace expansion oddity

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

Zsh expands an empty alternative in brace expansion into an empty string
argument (as if quoted), whereas bash expands it to nothing.  Note the
number of blank lines printed in each case below.

schaefer[501] bash2
[schaefer@toltec schaefer]$ printf "%s\n" {,}

[schaefer@toltec schaefer]$ printf "%s\n" x {,} y
x
y
[schaefer@toltec schaefer]$ exit
schaefer[502] printf "%s\n" x {,} y
x


y
schaefer[503] printf "%s\n" {,}


schaefer[504]