qt: fix 'opens in testnet mode when presented with a BIP-72 link with no fallback'

Passes tests:

```
$ ./bitcoin-qt 'bitcoin:?r=http://www.example.com/'
.. fixed the original problem - this launches mainnet.

$ ./bitcoin-qt 'bitcoin:mngeNQbTKnmaMbx8EXCYdwUbnt9JJD52cC'
.. launches testnet

$ ./bitcoin-qt -testnet 'bitcoin:1NXXeQRyMFFFRfyUix2o7mk1vhvk2Nxp78'
.. sanity check - launches mainnet.
```

Fixes #4355. Closes #4411.

Rebased-From: dd49e92

src/qt/paymentserver.cpp

@@ -191,7 +191,7 @@ bool PaymentServer::ipcParseCommandLine(int argc, char* argv[])
             savedPaymentRequests.append(arg);
 
             SendCoinsRecipient r;
-            if (GUIUtil::parseBitcoinURI(arg, &r))
+            if (GUIUtil::parseBitcoinURI(arg, &r) && !r.address.isEmpty())
             {
                 CBitcoinAddress address(r.address.toStdString());