[PM-10542] Handle TOTP codes with empty or whitespace-only keys

[KatherineInCode]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-10542

📔 Objective

When a TOTP was removed from a login entry that had one, the app was saving the TOTP as an empty string rather than nil; as well, if asked to generate a code for an empty string, it would assume it a valid key and generate it. This change adjusts this so that empty and whitespace-only strings coalesce into nil values. Some tests around this are also added.

⏰ Reminders before review

• Contributor guidelines followed
• All formatters and local linters executed and passed
• Written new unit and / or integration tests where applicable
• Protected functional changes with optionality (feature flags)
• Used internationalization (i18n) for all UI strings
• CI builds passed
• Communicated to DevOps any deployment requirements
• Updated any necessary documentation (Confluence, contributing docs) or informed the documentation team

🦮 Reviewer guidelines

• 👍 (:+1:) or similar for great changes
• 📝 (:memo:) or ℹ️ (:information_source:) for notes or general info
• ❓ (:question:) for questions
• 🤔 (:thinking:) or 💭 (:thought_balloon:) for more open inquiry that's not quite a confirmed issue and could potentially benefit from discussion
• 🎨 (:art:) for suggestions / improvements
• ❌ (:x:) or ⚠️ (:warning:) for more significant problems or concerns needing attention
• 🌱 (:seedling:) or ♻️ (:recycle:) for future improvements or indications of technical debt
• ⛏ (:pick:) for minor or nitpick changes

[github-actions[bot]]

Checkmarx One – Scan Summary & Details – a29157e9-5a23-496f-8bc2-237c90cc6532

No New Or Fixed Issues Found

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.50%. Comparing base (450808c) to head (c067fd7). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #835 +/- ## ======================================= Coverage 88.50% 88.50% ======================================= Files 599 599 Lines 30214 30214 ======================================= Hits 26742 26742 Misses 3472 3472 ```

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