BUG/MINOR: hlua: fix return with push nil on proxy check

BUG/MINOR: hlua: fix return with push nil on proxy check

hlua_check_proxy() may now return NULL if the target proxy instance has
been flagged for deletion. Thus, proxies method have been adjusted and
may push nil to report such case.

This patch fixes these error paths. When nil is pushed, 1 must be
returned instead of 0. This represents the count of pushed values on the
stack which can be retrieved by the caller.

No need to backport.