cleanup: return instead of fallthrough + improve comment (#844)

Return the TransactionSelector that is built instead of falling through to the end of
the method. The behavior does not change by this, but it makes it easier to read and
understand the code.

Fixes #830

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

@@ -463,7 +463,10 @@ TransactionSelector getTransactionSelector() {
             // Aborted error if the call that included the BeginTransaction option fails. The
             // Aborted error will cause the entire transaction to be retried, and the retry will use
             // a separate BeginTransaction RPC.
-            TransactionSelector.newBuilder()
+            // If tx.get() returns successfully, this.transactionId will also have been set to a
+            // valid value as the latter is always set when a transaction id is returned by a
+            // statement.
+            return TransactionSelector.newBuilder()
                 .setId(tx.get(waitForTransactionTimeoutMillis, TimeUnit.MILLISECONDS))
                 .build();
           }