Fix flip-flop operator possible children nodes count

One of the "flip-flop"'s operator nodes might be `nil`:

```bash
$ ruby-parse --34 -e '!(...(a == 1))'
(send
  (begin
    (eflipflop nil
      (begin
        (send
          (send nil :a) :==
          (int 1))))) :!)
```

```bash
$ docker run -it --rm rubylang/all-ruby env ALL_RUBY_SINCE=ruby-2.7.0 ./all-ruby -e '!(..(a==1))'
ruby-2.7.0
...
ruby-3.4.1
```

Author: viralpraxis

lib/rubocop/ast/traversal.rb

@@ -118,15 +118,15 @@ def children_count_check_code(range)
       many_node_children = %i[dstr dsym xstr regexp array hash pair
                               mlhs masgn or_asgn and_asgn rasgn mrasgn
                               undef alias args super yield or and
-                              while_post until_post iflipflop eflipflop
+                              while_post until_post
                               match_with_lvasgn begin kwbegin return
                               in_match match_alt break next
                               match_as array_pattern array_pattern_with_tail
                               hash_pattern const_pattern find_pattern
                               index indexasgn procarg0 kwargs]
       many_opt_node_children = %i[case rescue resbody ensure for when
                                   case_match in_pattern irange erange
-                                  match_pattern match_pattern_p]
+                                  match_pattern match_pattern_p iflipflop eflipflop]
 
       ### Callbacks for above
       def_callback no_children

spec/rubocop/ast/fixtures/code_examples.rb

@@ -1148,6 +1148,18 @@ class A < B
 #----
 if foo...bar; end
 
+#----
+!(...bar)
+
+#----
+!(..bar)
+
+#----
+!(bar...)
+
+#----
+!(bar..)
+
 #----
 !(foo...bar)