allowNull corresponds to default value `null`

[SOHELAHMED7]

Resolve below TODO:

File: src/lib/items/Attribute.php::toColumnSchema():

Conclusion:

It is fine to set defaultValue to null if allowNull is true.

If I create a table with multiple columns. And if a column (say "c2") has allowNull is true then when I try to insert (by raw SQL statement) a row without specifying column "c2" then its value will be automatically inserted as null.

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Part of #10

and

Part of cebe#180