MINOR: Fix TestVariantVector#testGetObject for parquet-variant 1.18.1

parquet-variant 1.18.1 added eager validation of the variant metadata
header in the Variant constructor. The test's placeholder metadata
bytes no longer decode as a structurally valid header, so
getObject() throws IllegalArgumentException. Use a valid
single-byte (version 1, empty dictionary) metadata header instead.

Author: jbonofre

arrow-variant/src/test/java/org/apache/arrow/variant/extension/TestVariantVector.java

@@ -769,7 +769,9 @@ void testGetObject() {
         ArrowBuf metadataBuf = allocator.buffer(10);
         ArrowBuf valueBuf = allocator.buffer(10)) {
 
-      byte[] metadata = new byte[] {1, 2};
+      // Version 1, empty dictionary (single header byte) - a structurally valid variant
+      // metadata, since getObject() eagerly parses the metadata header.
+      byte[] metadata = new byte[] {1};
       byte[] value = new byte[] {3, 4, 5};
       metadataBuf.setBytes(0, metadata);
       valueBuf.setBytes(0, value);