[stable-2.19] Bug-fix Check ownCloudClient nullability in NextcloudClient

[backportbot]

Backport of PR #1477

[github-actions]

SpotBugs

Category	Base	New
Bad practice	35	35
Correctness	34	34
Dodgy code	26	26
Internationalization	6	6
Malicious code vulnerability	49	49
Multithreaded correctness	3	3
Performance	8	8
Total	161	161

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 48.94%. Comparing base (682baac) to head (75924c6).
Report is 20 commits behind head on stable-2.19.

Files with missing lines	Patch %	Lines
.../main/java/com/nextcloud/common/NextcloudClient.kt	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##             stable-2.19    #1636   +/-   ##
==============================================
  Coverage          48.93%   48.94%           
  Complexity           996      996           
==============================================
  Files                206      206           
  Lines               7833     7836    +3     
  Branches            1019     1020    +1     
==============================================
+ Hits                3833     3835    +2     
- Misses              3450     3451    +1     
  Partials             550      550           

Files with missing lines	Coverage Δ
.../main/java/com/nextcloud/common/NextcloudClient.kt	50.49% <40.00%> (+0.49%)	⬆️