[HOTFIX] Do not throw NPE if spark.test.home is not set

`spark.test.home` was introduced in #1734. This is fine for SBT but is failing maven tests. Either way it shouldn't throw an NPE.

Author: Andrew Or <andrewor14@gmail.com>

Closes #1739 from andrewor14/fix-spark-test-home and squashes the following commits:

ce2624c [Andrew Or] Do not throw NPE if spark.test.home is not set