fix(config): replace external.ip=null with empty string in test configs

ConfigBeanFactory cannot bind HOCON null to String fields. Replace
external.ip=null with external.ip="" in 3 test config files. Empty
string has same semantics (no external IP configured). This allows
DiscoveryConfig sub-bean to auto-bind correctly.

Author: vividctrlalt

common/src/main/java/org/tron/core/config/args/NodeConfig.java

@@ -59,9 +59,7 @@ public class NodeConfig {
 
   public boolean isDiscoveryEnable() { return discovery.isEnable(); }
   public boolean isDiscoveryPersist() { return discovery.isPersist(); }
-  public String getDiscoveryExternalIp() {
-    return discovery.getExternal().getIp();
-  }
+  public String getDiscoveryExternalIp() { return discovery.getExternal().getIp(); }
   public String getShutdownBlockTime() { return shutdownBlockTime; }
   public long getShutdownBlockHeight() { return shutdownBlockHeight; }
   public long getShutdownBlockCount() { return shutdownBlockCount; }

framework/src/test/resources/config-localtest.conf

@@ -57,7 +57,7 @@ storage {
 node.discovery = {
   enable = true
   persist = true
-  external.ip = null
+  external.ip = ""
 }
 
 node.backup {

framework/src/test/resources/config-test-dbbackup.conf

@@ -60,7 +60,7 @@ storage {
 node.discovery = {
   enable = true
   persist = true
-  external.ip = null
+  external.ip = ""
 }
 
 node.backup {

framework/src/test/resources/config-test-index.conf

@@ -54,7 +54,7 @@ storage {
 node.discovery = {
   enable = true
   persist = true
-  external.ip = null
+  external.ip = ""
 }
 
 node {