Fixes potential NullPointerException on shard closing (#21515)

It was possible that in IndexService#closeShard(), an attempt
would be made to close the store where the store was not yet
initialized, throwing a NullPointerException.  This commit
ensures that we do not call close on a store that has not
yet been initialized.

core/src/main/java/org/elasticsearch/index/IndexService.java

@@ -406,7 +406,11 @@ private void closeShard(String reason, ShardId sId, IndexShard indexShard, Store
             }
         } finally {
             try {
-                store.close();
+                if (store != null) {
+                    store.close();
+                } else {
+                    logger.trace("[{}] store not initialized prior to closing shard, nothing to close", shardId);
+                }
             } catch (Exception e) {
                 logger.warn(
                     (Supplier<?>) () -> new ParameterizedMessage(