[joiner] check for non-zero joiner UDP port before electing a joiner router (#9445)

An invalid Discovery Response that carries no Joiner UDP Port TLV will
be accepted by the joiner, but we shouldn't qualify it as a valid
joiner router since we can only open a connection with a non-zero
port.

src/core/meshcop/joiner.cpp

@@ -261,7 +261,7 @@ void Joiner::HandleDiscoverResult(Mle::DiscoverScanner::ScanResult *aResult)
 {
     VerifyOrExit(mState == kStateDiscover);
 
-    if (aResult != nullptr)
+    if (aResult != nullptr && aResult->mJoinerUdpPort > 0)
     {
         SaveDiscoveredJoinerRouter(*aResult);
     }