API: Fix null pointer on not logged in attempt to Sudo

[zeripath]

If you attempt to sudo when you're not logged in the current implementation will cause a null pointer dereference. This PR protects the check to ctx.User with a ctx.IsSigned check preventing this behaviour.

Signed-off-by: Andrew Thornton art27@cantab.net

[codecov-io]

Codecov Report

Merging #5872 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5872      +/-   ##
==========================================
+ Coverage   37.84%   37.85%   +<.01%     
==========================================
  Files         328      328              
  Lines       48326    48326              
==========================================
+ Hits        18290    18292       +2     
+ Misses      27404    27401       -3     
- Partials     2632     2633       +1

Impacted Files	Coverage Δ
routers/api/v1/api.go	73.76% <100%> (ø)	⬆️
modules/process/manager.go	76.81% <0%> (-4.35%)	⬇️
modules/sync/unique_queue.go	89.28% <0%> (+10.71%)	⬆️
models/unit.go	14.28% <0%> (+14.28%)	⬆️

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