This allows partial-instantiating custom Null object for template Lookup<T>.
Before, this had to be handcoded per instantiation. Apparently I missed
adding one for AAT::ankr.lookupTable, so it was getting the wrong (generic)
null for Lookup object, which is wrong and unsafe.
Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=944346
Behdad Esfahbod