[SPARK-16606][CORE] Misleading warning for SparkContext.getOrCreate "WARN...

[SPARK-16606][CORE] Misleading warning for SparkContext.getOrCreate "WARN SparkContext: Use an existing SparkContext, some configuration may not take effect."

## What changes were proposed in this pull request?

SparkContext.getOrCreate shouldn't warn about ignored config if

- it wasn't ignored because a new context is created with it or
- no config was actually provided

## How was this patch tested?

Jenkins + existing tests.

Author: Sean Owen <sowen@cloudera.com>

Closes #14533 from srowen/SPARK-16606.

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -2262,9 +2262,10 @@ object SparkContext extends Logging {
     SPARK_CONTEXT_CONSTRUCTOR_LOCK.synchronized {
       if (activeContext.get() == null) {
         setActiveContext(new SparkContext(config), allowMultipleContexts = false)
-      }
-      if (config.getAll.nonEmpty) {
-        logWarning("Use an existing SparkContext, some configuration may not take effect.")
+      } else {
+        if (config.getAll.nonEmpty) {
+          logWarning("Using an existing SparkContext; some configuration may not take effect.")
+        }
       }
       activeContext.get()
     }
@@ -2281,7 +2282,12 @@ object SparkContext extends Logging {
    * even if multiple contexts are allowed.
    */
   def getOrCreate(): SparkContext = {
-    getOrCreate(new SparkConf())
+    SPARK_CONTEXT_CONSTRUCTOR_LOCK.synchronized {
+      if (activeContext.get() == null) {
+        setActiveContext(new SparkContext(), allowMultipleContexts = false)
+      }
+      activeContext.get()
+    }
   }
 
   /**