[libc++] Fix calling push_heap on an empty range in the push_heap benchmark (#227076)

std::push_heap requires a non-empty range because it inserts the element
at position `last - 1` (which isn't valid for an empty range) into the
heap. This didn't show up as a problem with libc++ since we have a
defensive implementation, but libstdc++ does not.
GitOrigin-RevId: fb00768dcf426f7dacda4af300d958368256e530