wallet_rpc_server: don't report integrated addresses for 0 payment ID

These are dummy ones

src/wallet/wallet_rpc_server.cpp

@@ -1173,8 +1173,11 @@ namespace tools
             crypto::hash payment_id;
             if(cryptonote::get_encrypted_payment_id_from_tx_extra_nonce(extra_nonce.nonce, payment_id8))
             {
-              desc.payment_id = epee::string_tools::pod_to_hex(payment_id8);
-              has_encrypted_payment_id = true;
+              if (payment_id8 != crypto::null_hash8)
+              {
+                desc.payment_id = epee::string_tools::pod_to_hex(payment_id8);
+                has_encrypted_payment_id = true;
+              }
             }
             else if (cryptonote::get_payment_id_from_tx_extra_nonce(extra_nonce.nonce, payment_id))
             {

tests/functional_tests/multisig.py

@@ -272,7 +272,7 @@ class MultisigTest():
           assert desc.amount_out == desc.amount_in - fee
           assert desc.ring_size == 11
           assert desc.unlock_time == 0
-          assert desc.payment_id == '0000000000000000'
+          assert not 'payment_id' in desc or desc.payment_id in ['', '0000000000000000']
           assert desc.change_amount == desc.amount_in - 1000000000000 - fee
           assert desc.change_address == self.wallet_address
           assert desc.fee == fee