[SPARK-15456][PYSPARK] Fixed PySpark shell context initialization when HiveConf not present

## What changes were proposed in this pull request?

When PySpark shell cannot find HiveConf, it will fallback to create a SparkSession from a SparkContext.  This fixes a bug caused by using a variable to SparkContext before it was initialized.

## How was this patch tested?

Manually starting PySpark shell and using the SparkContext

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #13237 from BryanCutler/pyspark-shell-session-context-SPARK-15456.