[SPARK-12234][SPARKR] Fix ```subset``` function error when only set ```select``` argument

Fix ```subset``` function error when only set ```select``` argument. Please refer to the [JIRA](https://issues.apache.org/jira/browse/SPARK-12234) about the error and how to reproduce it.

cc sun-rui felixcheung shivaram

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #10217 from yanboliang/spark-12234.