[SPARK-21588][SQL] SQLContext.getConf(key, null) should return null

## What changes were proposed in this pull request?

In SQLContext.get(key,null) for a key that is not defined in the conf, and doesn't have a default value defined, throws a NPE. Int happens only when conf has a value converter

Added null check on defaultValue inside SQLConf.getConfString to avoid calling entry.valueConverter(defaultValue)

## How was this patch tested?
Added unit test

Author: vinodkc <vinod.kc.in@gmail.com>

Closes #18852 from vinodkc/br_Fix_SPARK-21588.