sql/parser: fix the action for empty rules

A yacc-generated parser is a giant switch in a loop and the data
structure is a stack implemented as indexes into an array. Each
occurrence of `$$.val` is really `thestack[curpos].val`. Unless one
nils things out in the rule action the previous value remains.

(That's true of all yacc generators that don't provide destructors for
values. GNU Bison and Lemon provide destructors, for example, goyacc
and old Bison don't)

Release note (bug fix): the `WITH` operand of
import/export/backup/restore was sometimes processed improperly. This
has been corrected.

Release note (bug fix): common table expressions with `WITH` in larger
queries were sometimes processed improperly. This has been corrected.

pkg/sql/parser/sql.y

@@ -1824,7 +1824,10 @@ opt_with_options:
   {
     $$.val = $4.kvOptions()
   }
-| /* EMPTY */ {}
+| /* EMPTY */
+  {
+    $$.val = nil
+  }
 
 copy_from_stmt:
   COPY table_name opt_column_list FROM STDIN
@@ -5255,7 +5258,10 @@ opt_with_clause:
   {
     $$.val = $1.with()
   }
-| /* EMPTY */ {}
+| /* EMPTY */
+  {
+    $$.val = nil
+  }
 
 opt_table:
   TABLE {}