patch 8.2.2252: Vim9: crash when using lambda without return type in dict

Problem:    Vim9: crash when using lambda without return type in dict.
Solution:   Without a return type use t_unknown. (closes #7587)
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 4998471..4289069 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -2077,6 +2077,10 @@
       assert_equal(g:test_hash_dict, {one: 1, two: 2})
 
       assert_equal({['a a']: 1, ['b/c']: 2}, {'a a': 1, "b/c": 2})
+
+      var d = {a: () => 3, b: () => 7}
+      assert_equal(3, d.a())
+      assert_equal(7, d.b())
   END
   CheckDefAndScriptSuccess(lines)