Remove global checkpoint constraint

A constraint on the global checkpoint was inadvertently committed from
the inlining global checkpoint work. Namely, the constraint prevents the
global checkpoint from advancing to no ops performed, a situation that
can occur when shards are started but empty.

Author: jasontedor

core/src/main/java/org/elasticsearch/index/seqno/GlobalCheckpointTracker.java

@@ -175,7 +175,7 @@ private synchronized void updateGlobalCheckpointOnPrimary() {
                             globalCheckpoint);
             throw new IllegalStateException(message);
         }
-        if (minLocalCheckpoint >= 0 && globalCheckpoint != minLocalCheckpoint) {
+        if (globalCheckpoint != minLocalCheckpoint) {
             logger.trace("global checkpoint updated to [{}]", minLocalCheckpoint);
             globalCheckpoint = minLocalCheckpoint;
         }

core/src/test/java/org/elasticsearch/index/seqno/GlobalCheckpointTrackerTests.java

@@ -457,9 +457,9 @@ public void testUpdateAllocationIdsFromMaster() throws Exception {
      * allocation ID to the in-sync set and removing it from pending, the local checkpoint update that freed the thread waiting for the
      * local checkpoint to advance could miss updating the global checkpoint in a race if the the waiting thread did not add the allocation
      * ID to the in-sync set and remove it from the pending set before the local checkpoint updating thread executed the global checkpoint
-     * update. This test fails without an additional to {@link GlobalCheckpointTracker#updateGlobalCheckpointOnPrimary()} after removing
-     * the allocation ID from the pending set in {@link GlobalCheckpointTracker#markAllocationIdAsInSync(String, long)} (even if a call is
-     * added after notifying all waiters in {@link GlobalCheckpointTracker#updateLocalCheckpoint(String, long)}).
+     * update. This test fails without an additional call to {@link GlobalCheckpointTracker#updateGlobalCheckpointOnPrimary()} after
+     * removing the allocation ID from the pending set in {@link GlobalCheckpointTracker#markAllocationIdAsInSync(String, long)} (even if a
+     * call is added after notifying all waiters in {@link GlobalCheckpointTracker#updateLocalCheckpoint(String, long)}).
      *
      * @throws InterruptedException   if the main test thread was interrupted while waiting
      * @throws BrokenBarrierException if the barrier was broken while the main test thread was waiting