MDArray::read_as_string_array(): make it robust to null string pointer being returned by GDAL

[rouault]

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• [X] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

I can't really come with a unit test for that given what exists in the bindings, but this is definitely something that could happen for example with the MEM driver where a user could write an array with null strings in it.

[lnicola]

Thanks!