Alter SaplingNote::nullifier() to take a SaplingFullViewingKey

This means the API will work if you only have a SaplingExtendedSpendingKey,
as will be the case with ZIP 32.

src/gtest/test_sapling_note.cpp

@@ -49,7 +49,7 @@ TEST(SaplingNote, TestVectors)
 
     // Test nullifier
     SaplingSpendingKey spendingKey(sk);
-    ASSERT_EQ(note.nullifier(spendingKey, note_pos), nf);
+    ASSERT_EQ(note.nullifier(spendingKey.full_viewing_key(), note_pos), nf);
 }