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`.

test/list/plus.wren

@@ -0,0 +1,18 @@
+var a = [1, 2, 3]
+var b = [4, 5, 6]
+var c = a + b
+var d = a + (4..6)
+var e = a + []
+var f = [] + a
+var g = [] + []
+
+IO.print(a) // expect: [1, 2, 3]
+IO.print(b) // expect: [4, 5, 6]
+IO.print(c) // expect: [1, 2, 3, 4, 5, 6]
+IO.print(d) // expect: [1, 2, 3, 4, 5, 6]
+IO.print(e) // expect: [1, 2, 3]
+IO.print(f) // expect: [1, 2, 3]
+IO.print(g) // expect: []
+
+// Doesn't modify original list.
+IO.print(a) // expect: [1, 2, 3]