TEST: init unassigned gcp in testAcquireIndexCommit

The global checkpoint should be assigned to unassigned rather than 0. If
a single document is indexed and the global checkpoint is initialized
with 0, the first commit is safe which the test does not suppose.

Relates #28038

server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java

@@ -4342,7 +4342,7 @@ public void testConcurrentAppendUpdateAndRefresh() throws InterruptedException,
     public void testAcquireIndexCommit() throws Exception {
         IOUtils.close(engine, store);
         store = createStore();
-        final AtomicLong globalCheckpoint = new AtomicLong();
+        final AtomicLong globalCheckpoint = new AtomicLong(SequenceNumbers.UNASSIGNED_SEQ_NO);
         try (InternalEngine engine = createEngine(store, createTempDir(), globalCheckpoint::get)) {
             int numDocs = between(1, 20);
             for (int i = 0; i < numDocs; i++) {