Don't count successive "," as literal in brace expansion

This was highly surprising.

Fixes #3002.

tests/test1.in

@@ -15,11 +15,15 @@ logmsg Bracket expansion
 echo x-{1}
 echo x-{1,2}
 echo foo-{1,2{3,4}}
+
 echo foo-{} # literal "{}" expands to itself
 echo foo-{{},{}} # the inner "{}" expand to themselves, the outer pair expands normally.
 echo foo-{""} # still expands to foo-
 echo foo-{$undefinedvar} # still expands to nothing
 
+echo foo-{,,,} # four empty items in the braces.
+echo foo-{,\,,} # an empty item, a "," and an empty item.
+
 logmsg Escaped newlines
 echo foo\ bar
 echo foo\