patch 8.2.1374: Vim9: error for assigning empty list to script variable

Problem:    Vim9: error for assigning empty list to script variable.
Solution:   Use t_unknown for empty list member. (closes #6595)

Author: brammool

src/testdir/test_vim9_script.vim

@@ -2782,6 +2782,20 @@ def Test_let_type_check()
     let var: asdf
   END
   CheckScriptFailure(lines, 'E1010:')
+
+  lines =<< trim END
+    vim9script
+    let s:l: list<number>
+    s:l = []
+  END
+  CheckScriptSuccess(lines)
+
+  lines =<< trim END
+    vim9script
+    let s:d: dict<number>
+    s:d = {}
+  END
+  CheckScriptSuccess(lines)
 enddef
 
 def Test_forward_declaration()

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1374,
 /**/
     1373,
 /**/

src/vim9compile.c

@@ -502,22 +502,25 @@ typval2type(typval_T *tv, garray_T *type_gap)
     if (tv->v_type == VAR_STRING)
 	return &t_string;
 
-    if (tv->v_type == VAR_LIST
-	    && tv->vval.v_list != NULL
-	    && tv->vval.v_list->lv_first != NULL)
+    if (tv->v_type == VAR_LIST)
     {
+	if (tv->vval.v_list == NULL || tv->vval.v_list->lv_first == NULL)
+	    return &t_list_empty;
+
 	// Use the type of the first member, it is the most specific.
 	member_type = typval2type(&tv->vval.v_list->lv_first->li_tv, type_gap);
 	return get_list_type(member_type, type_gap);
     }
 
-    if (tv->v_type == VAR_DICT
-	    && tv->vval.v_dict != NULL
-	    && tv->vval.v_dict->dv_hashtab.ht_used > 0)
+    if (tv->v_type == VAR_DICT)
     {
 	dict_iterator_T iter;
 	typval_T	*value;
 
+	if (tv->vval.v_dict == NULL
+				   || tv->vval.v_dict->dv_hashtab.ht_used == 0)
+	    return &t_dict_empty;
+
 	// Use the type of the first value, it is the most specific.
 	dict_iterate_start(tv, &iter);
 	dict_iterate_next(&iter, &value);