When merging two schema definitions that both has Metadata::FROM_NULL as a result we are going to get a nullable string type without Metadata::FROM_NULL as it's being removed.

If that metadata is removed during merge, it wont be possible later to merge it properly with entry that is not created from null, which will cause the schema corruption.

In order to avoid that problem we need to detect if both merged sides has FROM_NULL metadata, merge them but without removing that metadata.