[SPARK-21418][SQL] NoSuchElementException: None.get in DataSourceScanExec with sun.io.serialization.extendedDebugInfo=true

## What changes were proposed in this pull request?

If no SparkConf is available to Utils.redact, simply don't redact.

## How was this patch tested?

Existing tests

Author: Sean Owen <sowen@cloudera.com>

Closes #19123 from srowen/SPARK-21418.