GH-859: Fix ARROW_STRUCT_CONFLICT_POLICY env var (#860)

## What's Changed

Do not specify default value when getting the
`arrow.struct.conflict.policy` property

Closes #859

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));
@@ -62,11 +64,11 @@ public abstract class AbstractStructVector extends AbstractContainerVector {
 
   /** Policy to determine how to react when duplicate columns are encountered. */
   public enum ConflictPolicy {
-    // Ignore the conflict and append the field. This is the default behaviour
+    // Ignore the conflict and append the field.
     CONFLICT_APPEND,
     // Keep the existing field and ignore the newer one.
     CONFLICT_IGNORE,
-    // Replace the existing field with the newer one.
+    // Replace the existing field with the newer one. This is the default behaviour
     CONFLICT_REPLACE,
     // Refuse the new field and error out.
     CONFLICT_ERROR