Fix NPE in tests.

Author: JoshRosen

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala

@@ -726,7 +726,7 @@ class CodegenContext {
     // be extremely expensive in certain cases, such as deeply-nested expressions which operate over
     // inputs with wide schemas. For more details on the performance issues that motivated this
     // flat, see SPARK-15680.
-    if (SparkEnv.get.conf.getBoolean("spark.sql.codegen.comments", false)) {
+    if (SparkEnv.get != null && SparkEnv.get.conf.getBoolean("spark.sql.codegen.comments", false)) {
       val name = freshName("c")
       val comment = if (text.contains("\n") || text.contains("\r")) {
         text.split("(\r\n)|\r|\n").mkString("/**\n * ", "\n * ", "\n */")