Fix Arrow export of constant null variants (#10973)

Fix Arrow export of constant VARIANT values containing SQL NULL or JSON
null. These previously failed with `cannot export Variant without
ParquetVariant core storage`.

The export preserves null masks and shredded values, including object
fields. Unsupported combinations of nested residual values and
overlapping shredding now return explicit errors instead of panicking or
losing data. Non-null constants and general chunked VARIANT export are
not covered.

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Signed-off-by: blaginin <github@blaginin.me>
Signed-off-by: Robert Kruszewski <github@robertk.io>
Co-authored-by: Robert Kruszewski <github@robertk.io>

Author: blaginin