fix(arrow/array): preserve run-end encoded empty values

[fallintoplace]

What

Make RunEndEncodedBuilder.AppendEmptyValue and AppendEmptyValues append the value builder empty value instead of a null.

Test

• go test ./arrow/array -run ^TestRunEndEncoded -count=1

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Since this is a draft, i'll hold off on further review until the conflicts are resolved and it's marked ready

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The empty-value behavior is correct for ordinary string values, but it introduces two correctness issues in mixed-builder and dictionary-backed cases. One silently converts a null to an empty value; the other creates an invalid dictionary reference that panics when read. Details and reproductions are inline.

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Blocking: After this call resets the unmarshalling state, the pending non-null empty value is indistinguishable from a previously unmarshalled null to UnmarshalOne. Calling AppendEmptyValue() followed by UnmarshalOne(null) produces runEnds=[2], values=[""], and logical values ["", ""]; the appended null is silently converted to an empty string. Expected run ends and values are [1,2] and ["", null]. Please track this state explicitly so a direct empty-value append cannot be mistaken for a null run, and add this sequence as a regression test.

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Major: This delegation creates an invalid result when the encoded type is a dictionary. dictionaryBuilder.AppendEmptyValue() appends index 0 without adding a dictionary entry. Consequently, REE<int16, dictionary<int8, string>> contains an index into a zero-length dictionary, and GetOneForMarshal(0) panics with arrow/array: index out of range. Please provide valid dictionary empty-value behavior or explicitly reject this case, with coverage.