Don't emit anything for a null in void context.
Don't emit anything for a null in void context.

We often use nqp::null() as a symbolic "null" op when optimizing bits
away. Make the code-gen cooperate with this technique.

cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp