fix: remove conditional guards in List.+ and handle empty list iteration in iterate native

The previous implementation of List.+ used conditional checks (`this.count > 0`, `that.count > 0`) before iterating, which caused the loop body to be skipped entirely for empty lists. This commit removes those guards, allowing the for-in loops to run unconditionally — the Wren VM's iterate/iterateValue protocol already handles the empty case correctly at the native level. Additionally, the `list_iterate` native in `wren_core.c` now returns `false` immediately when the list count is zero and the iterator argument is null, preventing an out-of-bounds index of 0 from being returned for empty lists. The test file `test/list/concat.wren` is renamed to `test/list/plus.wren` and extended with a check that the original list `a` remains unmodified after concatenation. A new test in `test/list/iterate.wren` verifies that iterating over an empty list returns `false`.

src/wren_core.c

@@ -53,18 +53,14 @@ static const char* libSource =
 "  }\n"
 "\n"
 "  + that {\n"
-"    var newList = []\n"
-"    if (this.count > 0) {\n"
-"      for (element in this) {\n"
-"        newList.add(element)\n"
-"      }\n"
+"    var result = []\n"
+"    for (element in this) {\n"
+"      result.add(element)\n"
 "    }\n"
-"    if (that is Range || that.count > 0) {\n"
-"      for (element in that) {\n"
-"        newList.add(element)\n"
-"      }\n"
+"    for (element in that) {\n"
+"      result.add(element)\n"
 "    }\n"
-"    return newList\n"
+"    return result\n"
 "  }\n"
 "}\n";
 
@@ -336,12 +332,17 @@ DEF_NATIVE(list_insert)
 
 DEF_NATIVE(list_iterate)
 {
+  ObjList* list = AS_LIST(args[0]);
+
   // If we're starting the iteration, return the first index.
-  if (IS_NULL(args[1])) RETURN_NUM(0);
+  if (IS_NULL(args[1]))
+  {
+    if (list->count == 0) RETURN_FALSE;
+    RETURN_NUM(0);
+  }
 
   if (!validateInt(vm, args, 1, "Iterator")) return PRIM_ERROR;
 
-  ObjList* list = AS_LIST(args[0]);
   int index = (int)AS_NUM(args[1]);
 
   // Stop if we're out of bounds.