Refactor: Centralize logic for checking if a share is password protected

[theCalcaholic]

Summary

Previously, there were numerous checks that check whether a share has an empty or null password scattered around the code base. This PR moves the logic for this into a single, central method on IShare.

This PR prepares and is a prerequisite for #61722.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[theCalcaholic]

@miaulalala This is the refactoring portion we were talking about.
From my perspective it is ready for review. :)

[theCalcaholic]

@come-nc Thanks for the review! I just force-pushed due to the missing signature.

[theCalcaholic]

@CarlSchwan @come-nc I just pushed a few fixes for mocked shares in various unit tests (which caused issues I missed before the CI pipelines were run).

[theCalcaholic]

@come-nc See this comment on the downstream PR #61722 regarding the failing CI jobs.

Let me know, if anything from my side is required (e.g. rebasing) :)

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