[5.0] Fix data_get() With ArrayAccess And Null Values

Follow-up to #6571 (see comments on commit 39db9e14d3235a29eb7751b12959d2b76fa82af4).

Also, tests with null values should be added, I leave it up to you as I'm afraid cluttering `testDataGet()` :sweat_smile:

**edit:** wrong, see notes below