chore(spanner): fallback for TransactionRunner

Author: harshachinta

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

@@ -89,17 +89,17 @@ public <T> T run(TransactionCallable<T> callable) {
 
   @Override
   public Timestamp getCommitTimestamp() {
-    return null;
+    return this.transactionRunner.getCommitTimestamp();
   }
 
   @Override
   public CommitResponse getCommitResponse() {
-    return null;
+    return this.transactionRunner.getCommitResponse();
   }
 
   @Override
   public TransactionRunner allowNestedTransaction() {
-    return null;
+    return this.transactionRunner.allowNestedTransaction();
   }
 }