chore(mysql): fix groupconcat impl

ibis/backends/sql/compilers/mysql.py

@@ -171,9 +171,13 @@ def visit_GroupConcat(self, op, *, arg, sep, where, order_by):
                 "Only string literal separators are supported"
             )
 
-        return super().visit_GroupConcat(
-            op, arg=arg, sep=sep, where=where, order_by=order_by
-        )
+        if where is not None:
+            arg = self.if_(where, arg, NULL)
+
+        if order_by:
+            arg = sge.Order(this=arg, expressions=order_by)
+
+        return sge.GroupConcat(this=arg, separator=sep)
 
     def visit_DayOfWeekIndex(self, op, *, arg):
         return (self.f.dayofweek(arg) + 5) % 7