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/gc/collector/semi_space.cc b/runtime/gc/collector/semi_space.cc
index dbf01d8..82d02e7 100644
--- a/runtime/gc/collector/semi_space.cc
+++ b/runtime/gc/collector/semi_space.cc
@@ -750,7 +750,7 @@
     SHARED_LOCKS_REQUIRED(Locks::heap_bitmap_lock_) {
   // All immune objects are assumed marked.
   if (from_space_->HasAddress(obj)) {
-    // Returns either the forwarding address or nullptr.
+    // Returns either the forwarding address or null.
     return GetForwardingAddressInFromSpace(obj);
   } else if (collect_from_space_only_ || immune_region_.ContainsObject(obj) ||
              to_space_->HasAddress(obj)) {