[perl #119351] fix regression in B::CV::GV

The fix for [perl #118525] introduced a regression in the behavior of
B::CV::GV.

Previously it would return a B::SPECIAL when the GV slot was NULL, the
incorrect fix changed it to return undef.

This change fixes B::CV::GV to return a B::SPECIAL for NULL for both
B::main_cv and for B::CV objects created from lexical subs.