refactor(config): drop replaceNullWithEmpty workaround

Address PR review from lxcmyf (2026-04-22, NodeConfig.java:370).

The `replaceNullWithEmpty` helper was originally a compatibility shim for
system-test configs that write `discovery.external.ip = null`. Rather than
generalizing the shim over all paths (the reviewer's concern), remove it
entirely and let ConfigBeanFactory fail fast on HOCON null values.

The auto-binding itself is the right safety net: any String-typed field that
receives `= null` from an external config surfaces at startup as
`ConfigBeanFactory.ValidationFailed` naming the exact path. That's the
correct long-term signal that the external config needs to use `""` or
remove the key, not to be papered over.

No in-repo configs write `= null`; external configs (system-test) should
update their own defaults.

Author: vividctrlalt

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

@@ -362,14 +362,9 @@ public static class DnsConfig {
   public static NodeConfig fromConfig(Config config) {
     Config section = config.getConfig("node");
 
-    // Replace null values with empty strings before bean binding.
-    // Some external configs (e.g. system-test) set "discovery.external.ip = null"
-    // which ConfigBeanFactory cannot bind to a String field.
-    // Note: hasPath() returns false for null values, use hasPathOrNull() instead.
-    section = replaceNullWithEmpty(section, "discovery.external.ip");
-    section = replaceNullWithEmpty(section, "trustNode");
-
-    // Auto-bind all fields and sub-beans
+    // Auto-bind all fields and sub-beans. ConfigBeanFactory fails fast with a
+    // descriptive path on any `= null` value — external configs that use the
+    // HOCON null keyword should fix their config rather than rely on silent coercion.
     NodeConfig nc = ConfigBeanFactory.create(section, NodeConfig.class);
 
     // isOpenFullTcpDisconnect: boolean "is" prefix breaks JavaBean pairing
@@ -500,14 +495,4 @@ private static String getString(Config config, String path, String defaultValue)
     return config.hasPath(path) ? config.getString(path) : defaultValue;
   }
 
-  // Replace a null config value with empty string so ConfigBeanFactory can bind it.
-  // hasPath() returns false for null values; hasPathOrNull() returns true.
-  private static Config replaceNullWithEmpty(Config config, String path) {
-    if (config.hasPathOrNull(path) && config.getIsNull(path)) {
-      return config.withValue(path,
-          com.typesafe.config.ConfigValueFactory.fromAnyRef(""));
-    }
-    return config;
-  }
-
 }