Fix: watchOnly logic for empty acctKeyPriv

[gap-editor]

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Description

closes #939
This PR updates the logic for determining the watchOnly state by replacing a check on acctInfo.acctKeyPriv != nil with len(acctInfo.acctKeyPriv) == 0.
This ensures correct detection of accounts without usable private keys — even if the key slice is non-nil but empty — preventing unintended signing attempts.

Relevant Go documentation:

• https://stackoverflow.com/questions/44305170/nil-slices-vs-non-nil-slices-vs-empty-slices-in-go-language

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
• Bug fixes contain tests triggering the bug to prevent regressions.

Code Style and Documentation

• The change is not [insubstantial](https://github.com/btcsuite/btcwallet/blob/master/docs/contribution_guidelines.md#substantial-contributions-only).
• The change obeys the [Code Documentation and Commenting](https://github.com/btcsuite/btcwallet/blob/master/docs/contribution_guidelines.md#code-documentation-and-commenting) guidelines, and lines wrap at 80.
• Commits follow the [Ideal Git Commit Structure](https://github.com/btcsuite/btcwallet/blob/master/docs/contribution_guidelines.md#ideal-git-commit-structure).
• Any new logging statements use an appropriate subsystem and logging level.

[guggero]

Can you please at least try to compile your code changes before submitting a PR? Using the GitHub UI for creating PR will almost never result in a PR that satisfies the pull request checklist. Perhaps you should focus on quality of PRs instead of quantity. 26 PRs in such a short time makes you look like a bot farming commits (didn't you even have "collecting commits" in your GitHub profile description a few days ago?).
So if you're serious about "contributing to open-source projects ", please don't generate more review work than what your "contributions" are actually worth.