wallet: don't witness viewing-key-only notes (fixes an abort on the next block)

A note found through an incoming viewing key only (z_importviewingkey)
has no full viewing key behind it. UpdateSaplingNullifierNoteMapWithTx
skips such notes, so they never get a nullifier. VerifyAndSetInitialWitness
treated "no nullifier" as "witness needs reseeding" on every call: it
cleared the note's witness, re-read the block from disk, reseeded it,
and then dereferenced the nullifier that never came. dragonxd aborted
in the txnotify thread (boost::optional "is_initialized" assert) on the
first block confirming a note to such an address, and again after every
restart, before the block index was flushed.

Such a note can't be spent, so it needs no witness. Skip it there; if
its spending key is imported later, it has a full viewing key and the
existing no-nullifier rule seeds it then. SaplingWitnessMinimumHeight
now takes the optional nullifier. A note without one is never extended
by BuildWitnessCache (its pending-note gate requires one), so it must
not lower the rebuild's start height. Every call site that worked before
passes a set nullifier and behaves as before.

The deref came in with the 2020 witness rework (6c48397cf). The new
gtest ViewingKeyOnlyNoteIsNotWitnessed fails on the old code (the note
reaches the reseed path).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/wallet/wallet.h

@@ -895,7 +895,7 @@ public:
 
 protected:
 
-    int SaplingWitnessMinimumHeight(const uint256& nullifier, int nWitnessHeight, int nMinimumHeight);
+    int SaplingWitnessMinimumHeight(const boost::optional<uint256>& nullifier, int nWitnessHeight, int nMinimumHeight);
 
     /**
      * pindex is the new tip being connected.