Allow older gramps versions to see (not crash) db schema 21

[dsblank]

Older versions of Gramps cannot see the new db format (21) but just crashes.

This PR:

1. adds a value column of type BLOB to metadata
2. writes the version value there

Fixes #13633

[Nick-Hall]

It looks like you didn't initialise the blob version for new databases.

I am getting the error: a bytes-like object is required, not 'NoneType'

[dsblank]

It looks like you didn't initialise the blob version for new databases.

Are you sure this went through the upgrade? You can force it by removing the json_data in the metadata table.

[dsblank]

Oh, I see... it only works on the upgrade. Needs to always work.

[dsblank]

@Nick-Hall, ok I believe I have all of the paths correct now. I tested a 6.0 newly create DB, opened in 5.2, and an upgraded 6.0 in 5.2. Both complained about the version 21.

[Nick-Hall]

Now I get a problem reading the name formats in the metadata table. It doesn't give me an error regarding incompatible versions.

Steps to reproduce:

1. Create a new database in gramps60
2. Import the example database
3. Open the database in gramps52

[dsblank]

I'm going to propose some changes to earlier version of Gramps that we check the version number before trying to read the rest of the metadata. Currently gramps5.2 , it checks the version AFTER it reads in a bunch of data. Let's move the check up before that, and release new versions.

[Nick-Hall]

Do we really want to release updates to old versions? How far back would we go?

People probably wouldn't upgrade and would still get the errors.

Can we add pickled empty lists to the rows still causing a problem?

[dsblank]

Can we add pickled empty lists to the rows still causing a problem?

I can try. I'll look at that this weekend.

[Nick-Hall]

@dsblank Don't spend too much time on this one. It mainly affects developers and doesn't corrupt the database.