Add consistent edge-case preconditions to sorting and searching demos

[Nullkernel]

Motivation

• Make interactive demos robust by validating user-provided sizes and avoiding common runtime errors.
• Ensure consistent behavior across all sorting and searching examples for negative, zero-length, and null/empty-array inputs.
• Document the intended edge-case semantics so maintainers and students understand how these examples behave on invalid or empty inputs.

Description

• Added input-size validation to every interactive main in Sorting-algorithms and Searching-algorithms to reject negative sizes and early-return on 0 with the message Nothing to sort/search and a closed scanner.
• Added algorithm-level guards (early-return or -1 for searches) for null/empty arrays such as if (arr == null || arr.length == 0) return; and adjusted helper methods accordingly.
• Implemented the requested radixsort(int[] arr) guard and protected Quicksort_Hoare recursion entry with if (arr == null || arr.length == 0) return; and if (low >= high) return;.
• Added a short comment block in each updated file describing the edge-case behavior for negative size, zero size, and null/empty arrays.

Testing

• Ran the repository smoke compile script bash scripts/compile_smoke.sh.
• Result: the compile smoke test passed and all Java files compiled successfully.

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Codex Task