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)
  
diff --git a/src/version.c b/src/version.c
index f080e68..d1bb9cc 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2252,
+/**/
     2251,
 /**/
     2250,
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 6ef2b5e..5602ff5 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -4837,7 +4837,8 @@
 	if (cctx->ctx_skip != SKIP_YES)
 	{
 	    stack_type = ((type_T **)stack->ga_data)[stack->ga_len - 1];
-	    if (check_return_type && cctx->ctx_ufunc->uf_ret_type == NULL)
+	    if (check_return_type && (cctx->ctx_ufunc->uf_ret_type == NULL
+				|| cctx->ctx_ufunc->uf_ret_type == &t_unknown))
 	    {
 		cctx->ctx_ufunc->uf_ret_type = stack_type;
 	    }
diff --git a/src/vim9type.c b/src/vim9type.c
index 3ff0762..d2dbc76 100644
--- a/src/vim9type.c
+++ b/src/vim9type.c
@@ -480,7 +480,10 @@
 	}
 	else if (expected->tt_type == VAR_FUNC)
 	{
-	    if (expected->tt_member != &t_unknown)
+	    // If the return type is unknown it can be anything, including
+	    // nothing, thus there is no point in checking.
+	    if (expected->tt_member != &t_unknown
+					    && actual->tt_member != &t_unknown)
 		ret = check_type(expected->tt_member, actual->tt_member,
 								     FALSE, 0);
 	    if (ret == OK && expected->tt_argcount != -1