fix(arcup): verify_checksum_file rejects valid checksum files without a trailing newline

[UzairRehman4]

Summary

• `read` returns a non-zero exit status when it hits EOF before a delimiter, even though it still populates the target variables from the partial final line
• `verify_checksum_file` treated that non-zero exit status as "file is empty" and aborted, so a valid `.sha256` file lacking a trailing newline was rejected
• Check whether `expected_checksum` is empty after the read instead of relying on `read`'s exit status; this still correctly rejects a genuinely empty checksum file

Fixes #420
Fixes #448

Test plan

• Added a regression case to `test_checksum_validation` in `test_arcup.sh` covering a valid checksum file with no trailing newline, and a genuinely empty checksum file
• Ran `bash arcup/test_arcup.sh` locally: all checksum-related tests pass (`ok - valid checksum file passes`, `ok - checksum filename mismatch fails`, `ok - valid checksum file without trailing newline passes`, `ok - empty checksum file fails`)

[github-actions]

Hi @UzairRehman4,

Thank you for your interest in contributing to Arc Node.

This PR has been automatically closed because you are not assigned to issue #420. We require contributors to be explicitly assigned to an issue before submitting a PR.

To contribute properly:

1. Comment on issue arcup: verify_checksum_file rejects a valid checksum file that has no trailing newline #420 requesting assignment
2. Wait for maintainer approval
3. Only submit a PR after you have been assigned

Please see our CONTRIBUTING.md for more details.

[github-actions]

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