[MERGE] avoid calling pp_null().

Several 'empty' ops like OP_NULL and OP_SCOPE call pp_null() at run-time
(which just returns). Attempts are made to strip such empty ops from the
op_next execution chain, but this has not been not complete. In
particular, ops at the head or tail of a sub-chain, or ops that rpeep()
has itself nulled, weren't being eliminated.

This merge avoids all calls to pp_null() in the test suite, apart from
those called via the constant folder (which is called before null op
elimination), and OP_REGCMAYBE (which isn't addressed here).