Add NULLVALUE template

Author: lucy66hw

tools/proto-convert/src/config/protobuf-schema-template/custom_message.mustache

@@ -30,3 +30,8 @@ message GeneralNumber {
     double double_value = 4;
   }
 }
+
+enum NullValue {
+  NULL_VALUE_UNSPECIFIED = 0;
+  NULL_VALUE_NULL = 1;
+}