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GH-39519: [Swift] Fix null count when using reader (#39520)

Currently the reader is not properly setting the null count when building an array from a stream.  This PR adds a fix for this.

* Closes: #39519

Authored-by: Alva Bandy <abandy@live.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>