[ruby/strscan] Sync missed commit

Syncs commit ruby/strscan@76b377a5d8.

ext/strscan/strscan.c

@@ -1502,7 +1502,9 @@ strscan_named_captures(VALUE self)
     named_captures_data data;
     data.self = self;
     data.captures = rb_hash_new();
-    onig_foreach_name(RREGEXP_PTR(p->regex), named_captures_iter, &data);
+    if (!RB_NIL_P(p->regex)) {
+        onig_foreach_name(RREGEXP_PTR(p->regex), named_captures_iter, &data);
+    }
 
     return data.captures;
 }

test/strscan/test_stringscanner.rb

@@ -775,6 +775,7 @@ module StringScannerTests
   def test_named_captures
     omit("not implemented on TruffleRuby") if ["truffleruby"].include?(RUBY_ENGINE)
     scan = StringScanner.new("foobarbaz")
+    assert_equal({}, scan.named_captures)
     assert_equal(9, scan.match?(/(?<f>foo)(?<r>bar)(?<z>baz)/))
     assert_equal({"f" => "foo", "r" => "bar", "z" => "baz"}, scan.named_captures)
   end