Fix ARROW_STRUCT_CONFLICT_POLICY env not work

Author: wForget

vector/src/main/java/org/apache/arrow/vector/complex/AbstractStructVector.java

@@ -46,11 +46,13 @@ public abstract class AbstractStructVector extends AbstractContainerVector {
   private ConflictPolicy conflictPolicy;
 
   static {
-    String conflictPolicyStr =
-        System.getProperty(STRUCT_CONFLICT_POLICY_JVM, ConflictPolicy.CONFLICT_REPLACE.toString());
+    String conflictPolicyStr = System.getProperty(STRUCT_CONFLICT_POLICY_JVM);
     if (conflictPolicyStr == null) {
       conflictPolicyStr = System.getenv(STRUCT_CONFLICT_POLICY_ENV);
     }
+    if (conflictPolicyStr == null) {
+      conflictPolicyStr = ConflictPolicy.CONFLICT_REPLACE.toString();
+    }
     ConflictPolicy conflictPolicy;
     try {
       conflictPolicy = ConflictPolicy.valueOf(conflictPolicyStr.toUpperCase(Locale.ROOT));