Improve Logging for Empty or Missing Socket Factory Configuration

In the current implementation of the NetUtils.getDefaultSocketFactory method, when the configuration parameter hadoop.rpc.socket.factory.class.default is not set or is empty, the method silently defaults to using the standard SocketFactory. However, this lack of logging can make it difficult to diagnose configuration issues. We propose enhancing the code with logging to warn users when this situation occurs.
 
Expected Behavior:
When the hadoop.rpc.socket.factory.class.default configuration parameter is missing or empty, a warning log should be generated, informing the user that the system is defaulting to the standard SocketFactory.
 
How-to-Fix:
We propose to expose this relationship by logging a warning when the hadoop.rpc.socket.factory.class.default parameter is missing or empty.