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`.

builtin/core.wren

@@ -10,17 +10,13 @@ class List {
   }
 
   + that {
-    var newList = []
-    if (this.count > 0) {
-      for (element in this) {
-        newList.add(element)
-      }
+    var result = []
+    for (element in this) {
+      result.add(element)
     }
-    if (that is Range || that.count > 0) {
-      for (element in that) {
-        newList.add(element)
-      }
+    for (element in that) {
+      result.add(element)
     }
-    return newList
+    return result
   }
 }