Support `undef` for `mrb_ary_splice()` instead of `[]`

When removing elements from an array, it is possible to avoid creating
an empty array.

Before this patch:

```c
mrb_ary_splice(mrb, ary, head, len, mrb_ary_new(mrb));
```

After this patch:

```c
mrb_ary_splice(mrb, ary, head, len, mrb_undef_value());
```

1 files changed, 4 insertions, 0 deletions

diff --git a/src/array.c b/src/array.c
index 8547cfff4..6e73bcd8e 100644
--- a/src/array.c
+++ b/src/array.c
@@ -732,6 +732,10 @@ mrb_ary_splice(mrb_state *mrb, mrb_value ary, mrb_int head, mrb_int len, mrb_val
       argv = ARY_PTR(r);
     }
   }
+  else if (mrb_undef_p(rpl)) {
+    argc = 0;
+    argv = NULL;
+  }
   else {
     argc = 1;
     argv = &rpl;