[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.

(cherry picked from commit 1ba967b2)
Signed-off-by: gatorsmile <gatorsmile@gmail.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -917,10 +917,12 @@ class SQLConf extends Serializable with Logging {
    * not set yet, return `defaultValue`.
    */
   def getConfString(key: String, defaultValue: String): String = {
-    val entry = sqlConfEntries.get(key)
-    if (entry != null && defaultValue != "<undefined>") {
-      // Only verify configs in the SQLConf object
-      entry.valueConverter(defaultValue)
+    if (defaultValue != null && defaultValue != "<undefined>") {
+      val entry = sqlConfEntries.get(key)
+      if (entry != null) {
+        // Only verify configs in the SQLConf object
+        entry.valueConverter(defaultValue)
+      }
     }
     Option(settings.get(key)).getOrElse(defaultValue)
   }

sql/core/src/test/scala/org/apache/spark/sql/internal/SQLConfSuite.scala

@@ -270,4 +270,15 @@ class SQLConfSuite extends QueryTest with SharedSQLContext {
     val e2 = intercept[AnalysisException](spark.conf.unset(SCHEMA_STRING_LENGTH_THRESHOLD.key))
     assert(e2.message.contains("Cannot modify the value of a static config"))
   }
+
+  test("SPARK-21588 SQLContext.getConf(key, null) should return null") {
+    withSQLConf(SQLConf.SHUFFLE_PARTITIONS.key -> "1") {
+      assert("1" == spark.conf.get(SQLConf.SHUFFLE_PARTITIONS.key, null))
+      assert("1" == spark.conf.get(SQLConf.SHUFFLE_PARTITIONS.key, "<undefined>"))
+    }
+
+    assert(spark.conf.getOption("spark.sql.nonexistent").isEmpty)
+    assert(null == spark.conf.get("spark.sql.nonexistent", null))
+    assert("<undefined>" == spark.conf.get("spark.sql.nonexistent", "<undefined>"))
+  }
 }