Avoid allocation when passing no keywords to anonymous kwrest methods

Thanks to the new semantics from [ruby-core:115808], `**nil` is now
equivalent to `**{}`. Since the only thing one could do with anonymous
keyword rest parameter is to delegate it with `**`, nil is just as good
as an empty hash. Using nil avoids allocating an empty hash.

This is particularly important for `...` methods since they now use
`**kwrest` under the hood after 4f77d8d. Most calls don't pass
keywords.

    Comparison:
                             fw_no_kw
                    post:   9816800.9 i/s
                     pre:   8570297.0 i/s - 1.15x  slower

benchmark/vm_method_splat_calls2.yml

@@ -7,6 +7,7 @@ prelude: |
   def anon_fw_to_named(*, **) named_arg_kw_splat(*, **) end
   def fw_to_named(...) named_arg_kw_splat(...) end
   def fw_to_anon_to_named(...) anon_fw_to_named(...) end
+  def fw_no_kw(...) named_arg_splat(...) end
   a = [1]
   kw = {y: 1}
 benchmark:
@@ -23,3 +24,4 @@ benchmark:
   fw_to_named_no_splat: "fw_to_named(1, y: 1)"
   fw_to_anon_to_named_splat: "fw_to_anon_to_named(*a, **kw)"
   fw_to_anon_to_named_no_splat: "fw_to_anon_to_named(1, y: 1)"
+  fw_no_kw: "fw_no_kw(1, 2)"

vm_args.c

@@ -474,10 +474,12 @@ args_setup_kw_parameters_from_kwsplat(rb_execution_context_t *const ec, const rb
 }
 
 static inline void
-args_setup_kw_rest_parameter(VALUE keyword_hash, VALUE *locals, int kw_flag)
+args_setup_kw_rest_parameter(VALUE keyword_hash, VALUE *locals, int kw_flag, bool anon_kwrest)
 {
     if (NIL_P(keyword_hash)) {
-        keyword_hash = rb_hash_new();
+        if (!anon_kwrest) {
+            keyword_hash = rb_hash_new();
+        }
     }
     else if (!(kw_flag & VM_CALL_KW_SPLAT_MUT)) {
         keyword_hash = rb_hash_dup(keyword_hash);
@@ -816,7 +818,8 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
         }
     }
     else if (ISEQ_BODY(iseq)->param.flags.has_kwrest) {
-        args_setup_kw_rest_parameter(keyword_hash, locals + ISEQ_BODY(iseq)->param.keyword->rest_start, kw_flag);
+        args_setup_kw_rest_parameter(keyword_hash, locals + ISEQ_BODY(iseq)->param.keyword->rest_start,
+            kw_flag, ISEQ_BODY(iseq)->param.flags.anon_kwrest);
     }
     else if (!NIL_P(keyword_hash) && RHASH_SIZE(keyword_hash) > 0 && arg_setup_type == arg_setup_method) {
         argument_kw_error(ec, iseq, "unknown", rb_hash_keys(keyword_hash));