Fix Internal Server Error when not logged in

[ghost]

When not logged in, accessing any Tasks page results in an Internal Server Error. This is because $userId is NULL, and SettingsService only accepts strings for user ids. As the error occurs during dependency injection of PageController, automatic redirects to the login page do not work.
This PR makes SettingsService accept NULL using nullable types, so Nextcloud can properly redirect the user to the login screen.

Error in log:

Error on web page:

[codecov]

Codecov Report

Merging #1138 into master will increase coverage by 2.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1138      +/-   ##
==========================================
+ Coverage   25.56%   27.82%   +2.26%     
==========================================
  Files          40       48       +8     
  Lines        2496     2609     +113     
  Branches      494      494              
==========================================
+ Hits          638      726      +88     
- Misses       1718     1743      +25     
  Partials      140      140              

[raimund-schluessler]

Thanks for the fix, looks good!