Give META_REDUCE_RIGHT behavior for 1-arg case

This aligns the behavior of `META_REDUCE_RIGHT` to that of
`META_REDUCE_LEFT` when handling 1-arg cases:

    > sub infix:<@> ($a, $b) is assoc<left> { [$a, $b] }
    > say [@] 10
    10

    > sub infix:<@> ($a, $b) is assoc<right> { [$a, $b] }
    > say [@] 10
    10

This addresses R#1705 (#1705).

src/core.c/metaops.rakumod

@@ -386,7 +386,11 @@ multi sub METAOP_REDUCE_RIGHT(\op) {
             ),
             nqp::if(
               $i,
-              op.(nqp::atpos(nqp::getattr($v,List,'$!reified'),0)),
+              nqp::if(
+                nqp::iseq_i($i, op.count),
+                op.(nqp::atpos(nqp::getattr($v,List,'$!reified'),0)),
+                nqp::atpos(nqp::getattr($v,List,'$!reified'),0)
+              ),
               op.()
             )
           )