use empty string, not null

Author: longbai

library/src/main/java/com/qiniu/android/http/Client.java

@@ -78,7 +78,7 @@ public com.squareup.okhttp.Response intercept(Chain chain) throws IOException {
 
                 com.squareup.okhttp.Response response = chain.proceed(request);
                 IpTag tag = (IpTag) request.tag();
-                String ip = null;
+                String ip = "";
                 try {
                    ip = chain.connection().getSocket().getRemoteSocketAddress().toString();
                 }catch (Exception e){