Enabled `useNullAsDefault` for SQlite3

closes #9587

- when `useNullAsDefault` was invented in knex 0.10, it was a breaking change, that update/insert etc no longer set's null as default if a field is missing
- at this time we thought it only affects our test env, because the test generator doesn't generate all fields
- but turned out the importer is affected as well e.g. you import a post with missing fields
- the importer doesn't iterate over all fields and checks if the field is present or not
- as this only happens with SQlite3, we should enable `useNullAsDefault` by default
- you can still disable this option if you want, but not recommended
- the reason why knex added this breaking change was that some applications want "undefined" as value
- this is not the case in Ghost, so it's fine to make use of the default null behaviour

core/server/data/db/connection.js

@@ -11,7 +11,7 @@ function configure(dbConfig) {
     var client = dbConfig.client;
 
     if (client === 'sqlite3') {
-        dbConfig.useNullAsDefault = dbConfig.useNullAsDefault || false;
+        dbConfig.useNullAsDefault = dbConfig.hasOwnProperty('useNullAsDefault') ? dbConfig.useNullAsDefault : true;
     }
 
     if (client === 'mysql') {