Refactor Workspace::get_user_workspaces

Summary:
With a void cache, the null value returned by the cache was passed to unserialize code.

While this issue was left aside to replace current caches classes by keruald/cache,
the function can already be split between cache/database query/build object methods.

This fixes the following issue:
[09-Nov-2025 16:47:46 UTC] PHP Deprecated:
unserialize(): Passing null to parameter #1 ($data) of type string is deprecated
in /var/51-wwwroot/obsidian/workspaces/src/Engines/Workspaces/Workspace.php on line 229

Test Plan: Check if the list in navbar header still contains workspaces.

Differential Revision: https://devcentral.nasqueron.org/D3862