Fix DefaultSslEngineFactory missing null check on close

patch by Abe Ratnofsky; reviewed by Andy Tolbert and Chris Lohfink for CASSANDRA-20001

Author: aratno

core/src/main/java/com/datastax/oss/driver/internal/core/ssl/DefaultSslEngineFactory.java

@@ -164,6 +164,6 @@ private ReloadingKeyManagerFactory buildReloadingKeyManagerFactory(DriverExecuti
 
   @Override
   public void close() throws Exception {
-    kmf.close();
+    if (kmf != null) kmf.close();
   }
 }