fix: SessionNotFound was not retried for AsyncTransactionManager (#552)

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/AsyncTransactionManagerImpl.java

@@ -66,7 +66,9 @@ public ApiFuture<Void> closeAsync() {
     if (txnState == TransactionState.STARTED) {
       res = rollbackAsync();
     }
-    txn.close();
+    if (txn != null) {
+      txn.close();
+    }
     return MoreObjects.firstNonNull(res, ApiFutures.<Void>immediateFuture(null));
   }
 
@@ -172,7 +174,7 @@ public ApiFuture<Void> apply(Empty input) throws Exception {
 
   @Override
   public TransactionContextFuture resetForRetryAsync() {
-    if (txn == null || !txn.isAborted() && txnState != TransactionState.ABORTED) {
+    if (txn == null || (!txn.isAborted() && txnState != TransactionState.ABORTED)) {
       throw new IllegalStateException(
           "resetForRetry can only be called if the previous attempt aborted");
     }