Fix #78694: Appending to a variant array causes segfault

`write_dimension` object handlers have to be able to handle `NULL`
`offset`s; for now we simply throw an exception instead of fol

Fix #78694: Appending to a variant array causes segfault

`write_dimension` object handlers have to be able to handle `NULL`
`offset`s; for now we simply throw an exception instead of following
the `NULL` pointer.

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