Avoid dereferencing a null field

utilities/document/document_db.cc

@@ -385,15 +385,15 @@ class SimpleSortedIndex : public Index {
       override {
     auto value = document.Get(field_);
     if (value == nullptr) {
-      // null
       if (!EncodeJSONPrimitive(JSONDocument(JSONDocument::kNull), key)) {
         assert(false);
       }
-    }
+    } else {
       if (!EncodeJSONPrimitive(*value, key)) {
         assert(false);
       }
     }
+  }
   virtual const Comparator* GetComparator() const override {
     return BytewiseComparator();
   }