chore(spanner): return in interceptor if txn is not created

Author: harshachinta

google-cloud-spanner/src/test/java/com/google/cloud/spanner/connection/ITAbstractSpannerTest.java

@@ -148,6 +148,9 @@ public void intercept(
             if (usingMultiplexedsession) {
               Field stateField = cls.getDeclaredField("txnState");
               stateField.setAccessible(true);
+              if(tx.getState() == null) {
+                return;
+              }
               tx.rollback();
               stateField.set(tx, TransactionState.ABORTED);
             } else {