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GH-42041: [Swift] Fix nullable type decoder issue (#42043)

### Rationale for this change

There is an issue when decoding nullable types.  The previous method of checking for nil values always returned false for nullable types due too the ArrowArray types being non nullable.

### What changes are included in this PR?
This PR adds a IsNull method to the ArrowDecoder to be used for null checks.  Also, a check for nullable types has been added to the Unkeyed decode method.

### Are these changes tested?
Yes, tests have been added/modified to test this fix.
* GitHub Issue: #42041

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