migrate: Do not remove non-HD keys from keys to be migrated as combo

Non-HD keys do not have an HD seed ID associated with them, so if this
value is the null value (all 0s), then we should not perform any seed ID
comparison that would result in excluding the keys from combo()
migration.

This changes the migration of non-HD wallets (or blank wallets with
imported private keys) to make a single combo() descriptors for the
non-HD/imported keys, rather than pk(), pkh(), sh(wpkh()), and wpkh()
descriptors for the keys.

src/wallet/scriptpubkeyman.cpp

@@ -1799,7 +1799,7 @@ std::optional<MigrationData> LegacyDataSPKM::MigrateToDescriptor()
                 keyid_it++;
                 continue;
             }
-            if (m_hd_chain.seed_id == meta.hd_seed_id || m_inactive_hd_chains.count(meta.hd_seed_id) > 0) {
+            if (!meta.hd_seed_id.IsNull() && (m_hd_chain.seed_id == meta.hd_seed_id || m_inactive_hd_chains.count(meta.hd_seed_id) > 0)) {
                 keyid_it = keyids.erase(keyid_it);
                 continue;
             }

test/functional/wallet_migration.py

@@ -1032,15 +1032,11 @@ def test_manual_keys_import(self):
         # There should be descriptors containing the imported key for: pk(), pkh(), sh(wpkh()), wpkh()
         key_origin = hash160(pubkey)[:4].hex()
         pubkey_hex = pubkey.hex()
-        pk_desc = descsum_create(f'pk([{key_origin}]{pubkey_hex})')
-        pkh_desc = descsum_create(f'pkh([{key_origin}]{pubkey_hex})')
-        sh_wpkh_desc = descsum_create(f'sh(wpkh([{key_origin}]{pubkey_hex}))')
-        wpkh_desc = descsum_create(f'wpkh([{key_origin}]{pubkey_hex})')
-        expected_descs = [pk_desc, pkh_desc, sh_wpkh_desc, wpkh_desc]
+        combo_desc = descsum_create(f"combo([{key_origin}]{pubkey_hex})")
 
         # Verify all expected descriptors were migrated
         migrated_desc = [item['desc'] for item in wallet.listdescriptors()['descriptors'] if pubkey.hex() in item['desc']]
-        assert_equal(expected_descs, migrated_desc)
+        assert_equal([combo_desc], migrated_desc)
         wallet.unloadwallet()
 
         ######################################################