SfcNshHeader builder doesn't include default value for UDP port

The SfcNshHeader class has a builder, as well as an "isValid()" method. The current isValid() method doesn't check for UDP port. This would be okay if there was a default port number provided, but there isn't a default method provided. We should include the UDP port in the isValid() method, and either include a default value for the UDP port or require it to be set explicitly in order for isValid() to return true.