Make CurrentContext return Handle<Context>::null

I noticed that ScopeIterator::CurrentContext returns an empty Handle
whereas functions like ScopeIterator::CurrentScopeInfo call
Handle<Context>::null() instead. This commit suggests changing this for
consistency.

Bug: 
Change-Id: I8735d655a8c0affeb6a18e74efe0d33bf6d5e899
Reviewed-on: https://chromium-review.googlesource.com/668440Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48057}

src/debug/debug-scopes.cc

@@ -408,7 +408,7 @@ Handle<Context> ScopeIterator::CurrentContext() {
   } else if (LastNestedScopeChain().scope_info->HasContext()) {
     return context_;
   } else {
-    return Handle<Context>();
+    return Handle<Context>::null();
   }
 }