Replace NULL with nullptr

Also fixed some lines that were too long, and a few other minor
details.

Change-Id: I6efba5fb6e03eb5d0a300fddb2a75bf8e2f175cb
diff --git a/runtime/native/java_lang_String.cc b/runtime/native/java_lang_String.cc
index 6afe83b..2d153d4 100644
--- a/runtime/native/java_lang_String.cc
+++ b/runtime/native/java_lang_String.cc
@@ -28,7 +28,7 @@
 
 static jint String_compareTo(JNIEnv* env, jobject javaThis, jobject javaRhs) {
   ScopedFastNativeObjectAccess soa(env);
-  if (UNLIKELY(javaRhs == NULL)) {
+  if (UNLIKELY(javaRhs == nullptr)) {
     ThrowNullPointerException("rhs == null");
     return -1;
   } else {