Avoid creating unnecessary empty arrays on splat.

But this changes requires `OP_ARYCAT` and `OP_ARYPUSH` to accept `nil`
as their first operand. Alternative VMs (e.g. `mruby/c`) that understand
mruby bytecode need to be updated.

mrbgems/mruby-compiler/core/codegen.c

@@ -956,7 +956,12 @@ gen_values(codegen_scope *s, node *t, int val, int extra)
         }
         else {
           pop_n(n);
-          genop_2(s, OP_ARRAY, cursp(), n);
+          if (n == 0 && is_splat) {
+            genop_1(s, OP_LOADNIL, cursp());
+          }
+          else {
+            genop_2(s, OP_ARRAY, cursp(), n);
+          }
           push();
           codegen(s, t->car, VAL);
           pop(); pop();

src/vm.c

@@ -2441,7 +2441,12 @@ RETRY_TRY_BLOCK:
 
     CASE(OP_ARYCAT, B) {
       mrb_value splat = mrb_ary_splat(mrb, regs[a+1]);
-      mrb_ary_concat(mrb, regs[a], splat);
+      if (mrb_nil_p(regs[a])) {
+        regs[a] = splat;
+      }
+      else {
+        mrb_ary_concat(mrb, regs[a], splat);
+      }
       mrb_gc_arena_restore(mrb, ai);
       NEXT;
     }