Add null checks and improve Javadoc in Strops

[alexstaicu35]

Summary

Harden Strops against null inputs and clarify its Javadoc so callers understand the contract.

Changes

• Throw IllegalArgumentException from reverse and isPalindrome when input is null
• Update Javadoc to document the null contract and the empty-string behaviour of isPalindrome
• Minor punctuation fixes in method descriptions

Notes

This introduces a behavioural change: previously a null argument would surface as a NullPointerException. Callers now receive an IllegalArgumentException with a descriptive message.

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changeset_id: 488cfdbd-a504-4565-b43e-b5fd717b3bb4

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