[MINOR] Fix build warnings

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to clean up build warnings.

Why are the changes needed?

BEFORE

$ swift build | grep warning
/Users/dongjoon/APACHE/spark-connect-swift/Sources/SparkConnect/ArrowReader.swift:301:27: warning: 'as' test is always true
    |                           `- warning: 'as' test is always true
/Users/dongjoon/APACHE/spark-connect-swift/Sources/SparkConnect/ArrowReader.swift:364:25: warning: 'as' test is always true
    |                         `- warning: 'as' test is always true
/Users/dongjoon/APACHE/spark-connect-swift/Sources/SparkConnect/ArrowReader.swift:301:27: warning: 'as' test is always true
    |                           `- warning: 'as' test is always true
/Users/dongjoon/APACHE/spark-connect-swift/Sources/SparkConnect/ArrowReader.swift:364:25: warning: 'as' test is always true
    |                         `- warning: 'as' test is always true
/Users/dongjoon/APACHE/spark-connect-swift/Sources/SparkConnect/ArrowReader.swift:301:27: warning: 'as' test is always true
    |                           `- warning: 'as' test is always true
/Users/dongjoon/APACHE/spark-connect-swift/Sources/SparkConnect/ArrowReader.swift:364:25: warning: 'as' test is always true
    |                         `- warning: 'as' test is always true

AFTER

$ swift build | grep warning

Does this PR introduce any user-facing change?

A code cleanup.

How was this patch tested?

Manual review.

Was this patch authored or co-authored using generative AI tooling?

No.

[dongjoon-hyun]

Could you review this minor Swift build warning fix when you have some time, @MaxGekk ?

[dongjoon-hyun]

Thank you, @MaxGekk and @viirya .

[dongjoon-hyun]

Merged to main.